[docs] Model sharding (#9521)

* flux shard

* feedback

docs/source/en/_toctree.yml

@@ -56,7 +56,7 @@
   - local: using-diffusers/overview_techniques
     title: Overview
   - local: training/distributed_inference
-    title: Distributed inference with multiple GPUs
+    title: Distributed inference
   - local: using-diffusers/merge_loras
     title: Merge LoRAs
   - local: using-diffusers/scheduler_features

docs/source/en/training/distributed_inference.md

@@ -10,7 +10,7 @@ an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express o
 specific language governing permissions and limitations under the License.
 -->
 
-# Distributed inference with multiple GPUs
+# Distributed inference
 
 On distributed setups, you can run inference across multiple GPUs with 🤗 [Accelerate](https://huggingface.co/docs/accelerate/index) or [PyTorch Distributed](https://pytorch.org/tutorials/beginner/dist_overview.html), which is useful for generating with multiple prompts in parallel.
 
@@ -109,3 +109,131 @@ torchrun run_distributed.py --nproc_per_node=2
 
 > [!TIP]
 > You can use `device_map` within a [`DiffusionPipeline`] to distribute its model-level components on multiple devices. Refer to the [Device placement](../tutorials/inference_with_big_models#device-placement) guide to learn more.
+
+## Model sharding
+
+Modern diffusion systems such as [Flux](../api/pipelines/flux) are very large and have multiple models. For example, [Flux.1-Dev](https://hf.co/black-forest-labs/FLUX.1-dev) is made up of two text encoders - [T5-XXL](https://hf.co/google/t5-v1_1-xxl) and [CLIP-L](https://hf.co/openai/clip-vit-large-patch14) - a [diffusion transformer](../api/models/flux_transformer), and a [VAE](../api/models/autoencoderkl). With a model this size, it can be challenging to run inference on consumer GPUs.
+
+Model sharding is a technique that distributes models across GPUs when the models don't fit on a single GPU. The example below assumes two 16GB GPUs are available for inference.
+
+Start by computing the text embeddings with the text encoders. Keep the text encoders on two GPUs by setting `device_map="balanced"`. The `balanced` strategy evenly distributes the model on all available GPUs. Use the `max_memory` parameter to allocate the maximum amount of memory for each text encoder on each GPU.
+
+> [!TIP]
+> **Only** load the text encoders for this step! The diffusion transformer and VAE are loaded in a later step to preserve memory.
+
+```py
+from diffusers import FluxPipeline
+import torch
+
+prompt = "a photo of a dog with cat-like look"
+
+pipeline = FluxPipeline.from_pretrained(
+    "black-forest-labs/FLUX.1-dev",
+    transformer=None,
+    vae=None,
+    device_map="balanced",
+    max_memory={0: "16GB", 1: "16GB"},
+    torch_dtype=torch.bfloat16
+)
+with torch.no_grad():
+    print("Encoding prompts.")
+    prompt_embeds, pooled_prompt_embeds, text_ids = pipeline.encode_prompt(
+        prompt=prompt, prompt_2=None, max_sequence_length=512
+    )
+```
+
+Once the text embeddings are computed, remove them from the GPU to make space for the diffusion transformer.
+
+```py
+import gc 
+
+def flush():
+    gc.collect()
+    torch.cuda.empty_cache()
+    torch.cuda.reset_max_memory_allocated()
+    torch.cuda.reset_peak_memory_stats()
+
+del pipeline.text_encoder
+del pipeline.text_encoder_2
+del pipeline.tokenizer
+del pipeline.tokenizer_2
+del pipeline
+
+flush()
+```
+
+Load the diffusion transformer next which has 12.5B parameters. This time, set `device_map="auto"` to automatically distribute the model across two 16GB GPUs. The `auto` strategy is backed by [Accelerate](https://hf.co/docs/accelerate/index) and available as a part of the [Big Model Inference](https://hf.co/docs/accelerate/concept_guides/big_model_inference) feature. It starts by distributing a model across the fastest device first (GPU) before moving to slower devices like the CPU and hard drive if needed. The trade-off of storing model parameters on slower devices is slower inference latency.
+
+```py
+from diffusers import FluxTransformer2DModel
+import torch 
+
+transformer = FluxTransformer2DModel.from_pretrained(
+    "black-forest-labs/FLUX.1-dev", 
+    subfolder="transformer",
+    device_map="auto",
+    torch_dtype=torch.bfloat16
+)
+```
+
+> [!TIP]
+> At any point, you can try `print(pipeline.hf_device_map)` to see how the various models are distributed across devices. This is useful for tracking the device placement of the models.
+
+Add the transformer model to the pipeline for denoising, but set the other model-level components like the text encoders and VAE to `None` because you don't need them yet.
+
+```py
+pipeline = FluxPipeline.from_pretrained(
+    "black-forest-labs/FLUX.1-dev", ,
+    text_encoder=None,
+    text_encoder_2=None,
+    tokenizer=None,
+    tokenizer_2=None,
+    vae=None,
+    transformer=transformer,
+    torch_dtype=torch.bfloat16
+)
+
+print("Running denoising.")
+height, width = 768, 1360
+latents = pipeline(
+    prompt_embeds=prompt_embeds,
+    pooled_prompt_embeds=pooled_prompt_embeds,
+    num_inference_steps=50,
+    guidance_scale=3.5,
+    height=height,
+    width=width,
+    output_type="latent",
+).images
+```
+
+Remove the pipeline and transformer from memory as they're no longer needed.
+
+```py
+del pipeline.transformer
+del pipeline
+
+flush()
+```
+
+Finally, decode the latents with the VAE into an image. The VAE is typically small enough to be loaded on a single GPU.
+
+```py
+from diffusers import AutoencoderKL
+from diffusers.image_processor import VaeImageProcessor
+import torch 
+
+vae = AutoencoderKL.from_pretrained(ckpt_id, subfolder="vae", torch_dtype=torch.bfloat16).to("cuda")
+vae_scale_factor = 2 ** (len(vae.config.block_out_channels))
+image_processor = VaeImageProcessor(vae_scale_factor=vae_scale_factor)
+
+with torch.no_grad():
+    print("Running decoding.")
+    latents = FluxPipeline._unpack_latents(latents, height, width, vae_scale_factor)
+    latents = (latents / vae.config.scaling_factor) + vae.config.shift_factor
+
+    image = vae.decode(latents, return_dict=False)[0]
+    image = image_processor.postprocess(image, output_type="pil")
+    image[0].save("split_transformer.png")
+```
+
+By selectively loading and unloading the models you need at a given stage and sharding the largest models across multiple GPUs, it is possible to run inference with large models on consumer GPUs.