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flow matching lcm scheduler (#11170)

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* add flow matching lcm scheduler
* stochastic sampling
* upscaling for scale-wise generation

* Apply style fixes

* Apply suggestions from code review

Co-authored-by: hlky <hlky@hlky.ac>

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: YiYi Xu <yixu310@gmail.com>
Co-authored-by: hlky <hlky@hlky.ac>
This commit is contained in:
Nikita Starodubcev
2025-04-13 00:14:57 +03:00
committed by GitHub
parent 0ef29355c9
commit ec0b2b3947
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# Copyright 2024 Stability AI, Katherine Crowson and The HuggingFace Team. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import math
from dataclasses import dataclass
from typing import List, Optional, Tuple, Union
import numpy as np
import torch
from ..configuration_utils import ConfigMixin, register_to_config
from ..utils import BaseOutput, is_scipy_available, logging
from ..utils.torch_utils import randn_tensor
from .scheduling_utils import SchedulerMixin
if is_scipy_available():
import scipy.stats
logger = logging.get_logger(__name__) # pylint: disable=invalid-name
@dataclass
class FlowMatchLCMSchedulerOutput(BaseOutput):
"""
Output class for the scheduler's `step` function output.
Args:
prev_sample (`torch.FloatTensor` of shape `(batch_size, num_channels, height, width)` for images):
Computed sample `(x_{t-1})` of previous timestep. `prev_sample` should be used as next model input in the
denoising loop.
"""
prev_sample: torch.FloatTensor
class FlowMatchLCMScheduler(SchedulerMixin, ConfigMixin):
"""
LCM scheduler for Flow Matching.
This model inherits from [`SchedulerMixin`] and [`ConfigMixin`]. Check the superclass documentation for the generic
methods the library implements for all schedulers such as loading and saving.
Args:
num_train_timesteps (`int`, defaults to 1000):
The number of diffusion steps to train the model.
shift (`float`, defaults to 1.0):
The shift value for the timestep schedule.
use_dynamic_shifting (`bool`, defaults to False):
Whether to apply timestep shifting on-the-fly based on the image resolution.
base_shift (`float`, defaults to 0.5):
Value to stabilize image generation. Increasing `base_shift` reduces variation and image is more consistent
with desired output.
max_shift (`float`, defaults to 1.15):
Value change allowed to latent vectors. Increasing `max_shift` encourages more variation and image may be
more exaggerated or stylized.
base_image_seq_len (`int`, defaults to 256):
The base image sequence length.
max_image_seq_len (`int`, defaults to 4096):
The maximum image sequence length.
invert_sigmas (`bool`, defaults to False):
Whether to invert the sigmas.
shift_terminal (`float`, defaults to None):
The end value of the shifted timestep schedule.
use_karras_sigmas (`bool`, defaults to False):
Whether to use Karras sigmas for step sizes in the noise schedule during sampling.
use_exponential_sigmas (`bool`, defaults to False):
Whether to use exponential sigmas for step sizes in the noise schedule during sampling.
use_beta_sigmas (`bool`, defaults to False):
Whether to use beta sigmas for step sizes in the noise schedule during sampling.
time_shift_type (`str`, defaults to "exponential"):
The type of dynamic resolution-dependent timestep shifting to apply. Either "exponential" or "linear".
scale_factors ('list', defaults to None)
It defines how to scale the latents at which predictions are made.
upscale_mode ('str', defaults to 'bicubic')
Upscaling method, applied if scale-wise generation is considered
"""
_compatibles = []
order = 1
@register_to_config
def __init__(
self,
num_train_timesteps: int = 1000,
shift: float = 1.0,
use_dynamic_shifting: bool = False,
base_shift: Optional[float] = 0.5,
max_shift: Optional[float] = 1.15,
base_image_seq_len: Optional[int] = 256,
max_image_seq_len: Optional[int] = 4096,
invert_sigmas: bool = False,
shift_terminal: Optional[float] = None,
use_karras_sigmas: Optional[bool] = False,
use_exponential_sigmas: Optional[bool] = False,
use_beta_sigmas: Optional[bool] = False,
time_shift_type: str = "exponential",
scale_factors: Optional[List[float]] = None,
upscale_mode: Optional[str] = "bicubic",
):
if self.config.use_beta_sigmas and not is_scipy_available():
raise ImportError("Make sure to install scipy if you want to use beta sigmas.")
if sum([self.config.use_beta_sigmas, self.config.use_exponential_sigmas, self.config.use_karras_sigmas]) > 1:
raise ValueError(
"Only one of `config.use_beta_sigmas`, `config.use_exponential_sigmas`, `config.use_karras_sigmas` can be used."
)
if time_shift_type not in {"exponential", "linear"}:
raise ValueError("`time_shift_type` must either be 'exponential' or 'linear'.")
timesteps = np.linspace(1, num_train_timesteps, num_train_timesteps, dtype=np.float32)[::-1].copy()
timesteps = torch.from_numpy(timesteps).to(dtype=torch.float32)
sigmas = timesteps / num_train_timesteps
if not use_dynamic_shifting:
# when use_dynamic_shifting is True, we apply the timestep shifting on the fly based on the image resolution
sigmas = shift * sigmas / (1 + (shift - 1) * sigmas)
self.timesteps = sigmas * num_train_timesteps
self._step_index = None
self._begin_index = None
self._shift = shift
self._init_size = None
self._scale_factors = scale_factors
self._upscale_mode = upscale_mode
self.sigmas = sigmas.to("cpu") # to avoid too much CPU/GPU communication
self.sigma_min = self.sigmas[-1].item()
self.sigma_max = self.sigmas[0].item()
@property
def shift(self):
"""
The value used for shifting.
"""
return self._shift
@property
def step_index(self):
"""
The index counter for current timestep. It will increase 1 after each scheduler step.
"""
return self._step_index
@property
def begin_index(self):
"""
The index for the first timestep. It should be set from pipeline with `set_begin_index` method.
"""
return self._begin_index
# Copied from diffusers.schedulers.scheduling_dpmsolver_multistep.DPMSolverMultistepScheduler.set_begin_index
def set_begin_index(self, begin_index: int = 0):
"""
Sets the begin index for the scheduler. This function should be run from pipeline before the inference.
Args:
begin_index (`int`):
The begin index for the scheduler.
"""
self._begin_index = begin_index
def set_shift(self, shift: float):
self._shift = shift
def set_scale_factors(self, scale_factors: list, upscale_mode):
"""
Sets scale factors for a scale-wise generation regime.
Args:
scale_factors (`list`):
The scale factors for each step
upscale_mode (`str`):
Upscaling method
"""
self._scale_factors = scale_factors
self._upscale_mode = upscale_mode
def scale_noise(
self,
sample: torch.FloatTensor,
timestep: Union[float, torch.FloatTensor],
noise: Optional[torch.FloatTensor] = None,
) -> torch.FloatTensor:
"""
Forward process in flow-matching
Args:
sample (`torch.FloatTensor`):
The input sample.
timestep (`int`, *optional*):
The current timestep in the diffusion chain.
Returns:
`torch.FloatTensor`:
A scaled input sample.
"""
# Make sure sigmas and timesteps have the same device and dtype as original_samples
sigmas = self.sigmas.to(device=sample.device, dtype=sample.dtype)
if sample.device.type == "mps" and torch.is_floating_point(timestep):
# mps does not support float64
schedule_timesteps = self.timesteps.to(sample.device, dtype=torch.float32)
timestep = timestep.to(sample.device, dtype=torch.float32)
else:
schedule_timesteps = self.timesteps.to(sample.device)
timestep = timestep.to(sample.device)
# self.begin_index is None when scheduler is used for training, or pipeline does not implement set_begin_index
if self.begin_index is None:
step_indices = [self.index_for_timestep(t, schedule_timesteps) for t in timestep]
elif self.step_index is not None:
# add_noise is called after first denoising step (for inpainting)
step_indices = [self.step_index] * timestep.shape[0]
else:
# add noise is called before first denoising step to create initial latent(img2img)
step_indices = [self.begin_index] * timestep.shape[0]
sigma = sigmas[step_indices].flatten()
while len(sigma.shape) < len(sample.shape):
sigma = sigma.unsqueeze(-1)
sample = sigma * noise + (1.0 - sigma) * sample
return sample
def _sigma_to_t(self, sigma):
return sigma * self.config.num_train_timesteps
def time_shift(self, mu: float, sigma: float, t: torch.Tensor):
if self.config.time_shift_type == "exponential":
return self._time_shift_exponential(mu, sigma, t)
elif self.config.time_shift_type == "linear":
return self._time_shift_linear(mu, sigma, t)
def stretch_shift_to_terminal(self, t: torch.Tensor) -> torch.Tensor:
r"""
Stretches and shifts the timestep schedule to ensure it terminates at the configured `shift_terminal` config
value.
Reference:
https://github.com/Lightricks/LTX-Video/blob/a01a171f8fe3d99dce2728d60a73fecf4d4238ae/ltx_video/schedulers/rf.py#L51
Args:
t (`torch.Tensor`):
A tensor of timesteps to be stretched and shifted.
Returns:
`torch.Tensor`:
A tensor of adjusted timesteps such that the final value equals `self.config.shift_terminal`.
"""
one_minus_z = 1 - t
scale_factor = one_minus_z[-1] / (1 - self.config.shift_terminal)
stretched_t = 1 - (one_minus_z / scale_factor)
return stretched_t
def set_timesteps(
self,
num_inference_steps: Optional[int] = None,
device: Union[str, torch.device] = None,
sigmas: Optional[List[float]] = None,
mu: Optional[float] = None,
timesteps: Optional[List[float]] = None,
):
"""
Sets the discrete timesteps used for the diffusion chain (to be run before inference).
Args:
num_inference_steps (`int`, *optional*):
The number of diffusion steps used when generating samples with a pre-trained model.
device (`str` or `torch.device`, *optional*):
The device to which the timesteps should be moved to. If `None`, the timesteps are not moved.
sigmas (`List[float]`, *optional*):
Custom values for sigmas to be used for each diffusion step. If `None`, the sigmas are computed
automatically.
mu (`float`, *optional*):
Determines the amount of shifting applied to sigmas when performing resolution-dependent timestep
shifting.
timesteps (`List[float]`, *optional*):
Custom values for timesteps to be used for each diffusion step. If `None`, the timesteps are computed
automatically.
"""
if self.config.use_dynamic_shifting and mu is None:
raise ValueError("`mu` must be passed when `use_dynamic_shifting` is set to be `True`")
if sigmas is not None and timesteps is not None:
if len(sigmas) != len(timesteps):
raise ValueError("`sigmas` and `timesteps` should have the same length")
if num_inference_steps is not None:
if (sigmas is not None and len(sigmas) != num_inference_steps) or (
timesteps is not None and len(timesteps) != num_inference_steps
):
raise ValueError(
"`sigmas` and `timesteps` should have the same length as num_inference_steps, if `num_inference_steps` is provided"
)
else:
num_inference_steps = len(sigmas) if sigmas is not None else len(timesteps)
self.num_inference_steps = num_inference_steps
# 1. Prepare default sigmas
is_timesteps_provided = timesteps is not None
if is_timesteps_provided:
timesteps = np.array(timesteps).astype(np.float32)
if sigmas is None:
if timesteps is None:
timesteps = np.linspace(
self._sigma_to_t(self.sigma_max), self._sigma_to_t(self.sigma_min), num_inference_steps
)
sigmas = timesteps / self.config.num_train_timesteps
else:
sigmas = np.array(sigmas).astype(np.float32)
num_inference_steps = len(sigmas)
# 2. Perform timestep shifting. Either no shifting is applied, or resolution-dependent shifting of
# "exponential" or "linear" type is applied
if self.config.use_dynamic_shifting:
sigmas = self.time_shift(mu, 1.0, sigmas)
else:
sigmas = self.shift * sigmas / (1 + (self.shift - 1) * sigmas)
# 3. If required, stretch the sigmas schedule to terminate at the configured `shift_terminal` value
if self.config.shift_terminal:
sigmas = self.stretch_shift_to_terminal(sigmas)
# 4. If required, convert sigmas to one of karras, exponential, or beta sigma schedules
if self.config.use_karras_sigmas:
sigmas = self._convert_to_karras(in_sigmas=sigmas, num_inference_steps=num_inference_steps)
elif self.config.use_exponential_sigmas:
sigmas = self._convert_to_exponential(in_sigmas=sigmas, num_inference_steps=num_inference_steps)
elif self.config.use_beta_sigmas:
sigmas = self._convert_to_beta(in_sigmas=sigmas, num_inference_steps=num_inference_steps)
# 5. Convert sigmas and timesteps to tensors and move to specified device
sigmas = torch.from_numpy(sigmas).to(dtype=torch.float32, device=device)
if not is_timesteps_provided:
timesteps = sigmas * self.config.num_train_timesteps
else:
timesteps = torch.from_numpy(timesteps).to(dtype=torch.float32, device=device)
# 6. Append the terminal sigma value.
# If a model requires inverted sigma schedule for denoising but timesteps without inversion, the
# `invert_sigmas` flag can be set to `True`. This case is only required in Mochi
if self.config.invert_sigmas:
sigmas = 1.0 - sigmas
timesteps = sigmas * self.config.num_train_timesteps
sigmas = torch.cat([sigmas, torch.ones(1, device=sigmas.device)])
else:
sigmas = torch.cat([sigmas, torch.zeros(1, device=sigmas.device)])
self.timesteps = timesteps
self.sigmas = sigmas
self._step_index = None
self._begin_index = None
def index_for_timestep(self, timestep, schedule_timesteps=None):
if schedule_timesteps is None:
schedule_timesteps = self.timesteps
indices = (schedule_timesteps == timestep).nonzero()
# The sigma index that is taken for the **very** first `step`
# is always the second index (or the last index if there is only 1)
# This way we can ensure we don't accidentally skip a sigma in
# case we start in the middle of the denoising schedule (e.g. for image-to-image)
pos = 1 if len(indices) > 1 else 0
return indices[pos].item()
def _init_step_index(self, timestep):
if self.begin_index is None:
if isinstance(timestep, torch.Tensor):
timestep = timestep.to(self.timesteps.device)
self._step_index = self.index_for_timestep(timestep)
else:
self._step_index = self._begin_index
def step(
self,
model_output: torch.FloatTensor,
timestep: Union[float, torch.FloatTensor],
sample: torch.FloatTensor,
generator: Optional[torch.Generator] = None,
return_dict: bool = True,
) -> Union[FlowMatchLCMSchedulerOutput, Tuple]:
"""
Predict the sample from the previous timestep by reversing the SDE. This function propagates the diffusion
process from the learned model outputs (most often the predicted noise).
Args:
model_output (`torch.FloatTensor`):
The direct output from learned diffusion model.
timestep (`float`):
The current discrete timestep in the diffusion chain.
sample (`torch.FloatTensor`):
A current instance of a sample created by the diffusion process.
generator (`torch.Generator`, *optional*):
A random number generator.
return_dict (`bool`):
Whether or not to return a [`~schedulers.scheduling_flow_match_lcm.FlowMatchLCMSchedulerOutput`] or
tuple.
Returns:
[`~schedulers.scheduling_flow_match_lcm.FlowMatchLCMSchedulerOutput`] or `tuple`:
If return_dict is `True`, [`~schedulers.scheduling_flow_match_lcm.FlowMatchLCMSchedulerOutput`] is
returned, otherwise a tuple is returned where the first element is the sample tensor.
"""
if (
isinstance(timestep, int)
or isinstance(timestep, torch.IntTensor)
or isinstance(timestep, torch.LongTensor)
):
raise ValueError(
(
"Passing integer indices (e.g. from `enumerate(timesteps)`) as timesteps to"
" `FlowMatchLCMScheduler.step()` is not supported. Make sure to pass"
" one of the `scheduler.timesteps` as a timestep."
),
)
if self._scale_factors and self._upscale_mode and len(self.timesteps) != len(self._scale_factors) + 1:
raise ValueError(
"`_scale_factors` should have the same length as `timesteps` - 1, if `_scale_factors` are set."
)
if self._init_size is None or self.step_index is None:
self._init_size = model_output.size()[2:]
if self.step_index is None:
self._init_step_index(timestep)
# Upcast to avoid precision issues when computing prev_sample
sample = sample.to(torch.float32)
sigma = self.sigmas[self.step_index]
sigma_next = self.sigmas[self.step_index + 1]
x0_pred = sample - sigma * model_output
if self._scale_factors and self._upscale_mode:
if self._step_index < len(self._scale_factors):
size = [round(self._scale_factors[self._step_index] * size) for size in self._init_size]
x0_pred = torch.nn.functional.interpolate(x0_pred, size=size, mode=self._upscale_mode)
noise = randn_tensor(x0_pred.shape, generator=generator, device=x0_pred.device, dtype=x0_pred.dtype)
prev_sample = (1 - sigma_next) * x0_pred + sigma_next * noise
# upon completion increase step index by one
self._step_index += 1
# Cast sample back to model compatible dtype
prev_sample = prev_sample.to(model_output.dtype)
if not return_dict:
return (prev_sample,)
return FlowMatchLCMSchedulerOutput(prev_sample=prev_sample)
# Copied from diffusers.schedulers.scheduling_euler_discrete.EulerDiscreteScheduler._convert_to_karras
def _convert_to_karras(self, in_sigmas: torch.Tensor, num_inference_steps) -> torch.Tensor:
"""Constructs the noise schedule of Karras et al. (2022)."""
# Hack to make sure that other schedulers which copy this function don't break
# TODO: Add this logic to the other schedulers
if hasattr(self.config, "sigma_min"):
sigma_min = self.config.sigma_min
else:
sigma_min = None
if hasattr(self.config, "sigma_max"):
sigma_max = self.config.sigma_max
else:
sigma_max = None
sigma_min = sigma_min if sigma_min is not None else in_sigmas[-1].item()
sigma_max = sigma_max if sigma_max is not None else in_sigmas[0].item()
rho = 7.0 # 7.0 is the value used in the paper
ramp = np.linspace(0, 1, num_inference_steps)
min_inv_rho = sigma_min ** (1 / rho)
max_inv_rho = sigma_max ** (1 / rho)
sigmas = (max_inv_rho + ramp * (min_inv_rho - max_inv_rho)) ** rho
return sigmas
# Copied from diffusers.schedulers.scheduling_euler_discrete.EulerDiscreteScheduler._convert_to_exponential
def _convert_to_exponential(self, in_sigmas: torch.Tensor, num_inference_steps: int) -> torch.Tensor:
"""Constructs an exponential noise schedule."""
# Hack to make sure that other schedulers which copy this function don't break
# TODO: Add this logic to the other schedulers
if hasattr(self.config, "sigma_min"):
sigma_min = self.config.sigma_min
else:
sigma_min = None
if hasattr(self.config, "sigma_max"):
sigma_max = self.config.sigma_max
else:
sigma_max = None
sigma_min = sigma_min if sigma_min is not None else in_sigmas[-1].item()
sigma_max = sigma_max if sigma_max is not None else in_sigmas[0].item()
sigmas = np.exp(np.linspace(math.log(sigma_max), math.log(sigma_min), num_inference_steps))
return sigmas
# Copied from diffusers.schedulers.scheduling_euler_discrete.EulerDiscreteScheduler._convert_to_beta
def _convert_to_beta(
self, in_sigmas: torch.Tensor, num_inference_steps: int, alpha: float = 0.6, beta: float = 0.6
) -> torch.Tensor:
"""From "Beta Sampling is All You Need" [arXiv:2407.12173] (Lee et. al, 2024)"""
# Hack to make sure that other schedulers which copy this function don't break
# TODO: Add this logic to the other schedulers
if hasattr(self.config, "sigma_min"):
sigma_min = self.config.sigma_min
else:
sigma_min = None
if hasattr(self.config, "sigma_max"):
sigma_max = self.config.sigma_max
else:
sigma_max = None
sigma_min = sigma_min if sigma_min is not None else in_sigmas[-1].item()
sigma_max = sigma_max if sigma_max is not None else in_sigmas[0].item()
sigmas = np.array(
[
sigma_min + (ppf * (sigma_max - sigma_min))
for ppf in [
scipy.stats.beta.ppf(timestep, alpha, beta)
for timestep in 1 - np.linspace(0, 1, num_inference_steps)
]
]
)
return sigmas
def _time_shift_exponential(self, mu, sigma, t):
return math.exp(mu) / (math.exp(mu) + (1 / t - 1) ** sigma)
def _time_shift_linear(self, mu, sigma, t):
return mu / (mu + (1 / t - 1) ** sigma)
def __len__(self):
return self.config.num_train_timesteps