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# TorchTPU

[TorchTPU](https://github.com/google-pytorch/torch_tpu/) is a PyTorch backend for Google's Tensor Processing Units
(TPUs), which lets you run Diffusers pipelines on Cloud TPUs (v6e, v7x) with minimal code changes.

Two execution modes are available:

| Mode | Constant | How to activate | Notes |
|---|---|---|---|
| Strict eager (default) | `EagerMode.DEFER_NEVER` | `pipe.to("tpu")` | Operations dispatched one at a time, asynchronous |
| Compile | — | `torch.compile(module, backend="tpu")` | AOT compilation with `TpuBackend` |

Follow the [TorchTPU installation guide](https://github.com/google-pytorch/torch_tpu/). Once installed, `import torch`
loads it automatically and registers the `"tpu"` device, so `pipe.to("tpu")` is the only change needed. Add
`import torch_tpu` only if you disabled backend autoloading with `TORCH_DEVICE_BACKEND_AUTOLOAD=0`.

## Eager mode

FLUX.1-schnell doesn't fit on a single v6e chip all at once, so use [`~DiffusionPipeline.enable_model_cpu_offload`] to
move each model to the TPU only while it runs. It detects the `"tpu"` device automatically.

```python
import torch

from diffusers import FluxPipeline

pipe = FluxPipeline.from_pretrained("black-forest-labs/FLUX.1-schnell", dtype=torch.bfloat16)
pipe.enable_model_cpu_offload()

image = pipe(
    prompt="a golden retriever surfing a wave, photorealistic",
    height=1024,
    width=1024,
    num_inference_steps=4,
    guidance_scale=0.0,
).images[0]

image.save("output.png")
```

If a model is too large for a single chip, or you have several chips and want lower latency, shard the models
across chips instead. See the [Tensor parallelism](#tensor-parallelism) section.

## Compiled mode

TorchTPU registers `"tpu"` as a `torch.compile` backend name (`TpuBackend` under the hood), so
components compile like any other `torch.compile` target. The first
call (warmup) is slow because it compiles; later calls with the same shapes reuse the compiled graph.

> [!IMPORTANT]
> TorchTPU requires **static shapes**, so pass `dynamic=False`. A new `height` or `width` compiles again for that
> shape, once; shapes already seen are reused. Changing `num_inference_steps` doesn't recompile.

When the whole pipeline fits on one chip, move it to the TPU and compile the full transformer. Stable Diffusion 3.5
Medium (~15GB in bf16) fits on a single v6e chip.

```python
import torch

from diffusers import StableDiffusion3Pipeline

pipe = StableDiffusion3Pipeline.from_pretrained("stabilityai/stable-diffusion-3.5-medium", dtype=torch.bfloat16)
pipe.to("tpu")
pipe.transformer.compile(backend="tpu", fullgraph=True, dynamic=False)

# Warmup — triggers static graph compilation.
pipe(prompt="warmup", height=1024, width=1024, num_inference_steps=40, guidance_scale=4.5)

# Later calls with the same shapes reuse the compiled graph.
image = pipe(
    prompt="a golden retriever surfing a wave, photorealistic",
    height=1024,
    width=1024,
    num_inference_steps=40,
    guidance_scale=4.5,
).images[0]

image.save("output.png")
```

If the pipeline doesn't fit on one chip:

- With several chips, shard it with [tensor parallelism](#tensor-parallelism) instead. Everything stays on the TPU,
  and `pipe.transformer.compile(...)` works the same way on the sharded transformer.
- With [`~DiffusionPipeline.enable_model_cpu_offload`], the offload hooks can't be traced by `torch.compile`. Compile
  only the transformer's repeated blocks instead, with
  `pipe.transformer.compile_repeated_blocks(backend="tpu", fullgraph=True, dynamic=False)`.

## Tensor parallelism

Shard models too large for one chip across several. FLUX.2-dev's text encoder (~48GB) and transformer (~64GB) each
exceed a single chip, so the example below shards both:

- the transformer with [`TensorParallelConfig`], passed to the `parallel_config` argument of [`~ModelMixin.from_pretrained`]. Each rank reads only its own slice of every sharded weight, so the full model is never materialized. For general TP details (`_tp_plan`, colwise/rowwise), see the [Tensor parallelism](../training/distributed_inference#tensor-parallelism) guide.
- the text encoder with Transformers' own [tensor parallelism](https://huggingface.co/docs/transformers/perf_infer_gpu_multi), passing a [`~transformers.DistributedConfig`] and the same mesh.

On TPU, initialize the process group with `backend="tpu_dist"` and build the mesh with `DeviceMesh("tpu", ...)`.

```python
import torch
import torch.distributed as dist
from torch.distributed.device_mesh import DeviceMesh
from transformers import DistributedConfig, Mistral3ForConditionalGeneration

from diffusers import Flux2Pipeline, Flux2Transformer2DModel, TensorParallelConfig

dist.init_process_group(backend="tpu_dist")
mesh = DeviceMesh("tpu", list(range(dist.get_world_size())))

repo_id = "black-forest-labs/FLUX.2-dev"
text_encoder = Mistral3ForConditionalGeneration.from_pretrained(
    repo_id,
    subfolder="text_encoder",
    dtype=torch.bfloat16,
    distributed_config=DistributedConfig(tp_plan="auto"),
    device_mesh=mesh,
)
transformer = Flux2Transformer2DModel.from_pretrained(
    repo_id, subfolder="transformer", dtype=torch.bfloat16, parallel_config=TensorParallelConfig(mesh=mesh)
)
pipe = Flux2Pipeline.from_pretrained(
    repo_id, text_encoder=text_encoder, transformer=transformer, dtype=torch.bfloat16
)
pipe.vae.to("tpu")

image = pipe(
    prompt="a golden retriever surfing a wave, photorealistic",
    num_inference_steps=28,
    generator=torch.Generator("cpu").manual_seed(0),
).images[0]
if dist.get_rank() == 0:
    image.save("output.png")
```

Launch one process per chip. Set `--nproc_per_node` to use all the number of TPU chips on your host.

```bash
eval $(python -m torch_tpu._internal.distributed.launchers.singlehost_wrapper | sed 's/^/export /')
torchrun --nproc_per_node=8 flux2_tp.py
```

> [!WARNING]
> The launch command above uses a TorchTPU internal module (`torch_tpu._internal`), not a stable API, and is expected
> to change in a future TorchTPU release. Check the [TorchTPU repository](https://github.com/google-pytorch/torch_tpu/)
> for the current way to launch multi-chip jobs.