[docs] improve distributed inference cp docs. (#12810)

* improve distributed inference cp docs.

* Apply suggestions from code review

Co-authored-by: Steven Liu <[email protected]>

---------

Co-authored-by: Steven Liu <[email protected]>

Author: sayakpaul

docs/source/en/training/distributed_inference.md

@@ -237,6 +237,8 @@ By selectively loading and unloading the models you need at a given stage and sh
 
 Use [`~ModelMixin.set_attention_backend`] to switch to a more optimized attention backend. Refer to this [table](../optimization/attention_backends#available-backends) for a complete list of available backends.
 
+Most attention backends are compatible with context parallelism. Open an [issue](https://github.com/huggingface/diffusers/issues/new) if a backend is not compatible.
+
 ### Ring Attention
 
 Key (K) and value (V) representations communicate between devices using [Ring Attention](https://huggingface.co/papers/2310.01889). This ensures each split sees every other token's K/V. Each GPU computes attention for its local K/V and passes it to the next GPU in the ring. No single GPU holds the full sequence, which reduces communication latency.
@@ -245,40 +247,60 @@ Pass a [`ContextParallelConfig`] to the `parallel_config` argument of the transf
 
 ```py
 import torch
-from diffusers import AutoModel, QwenImagePipeline, ContextParallelConfig
-
-try:
-    torch.distributed.init_process_group("nccl")
-    rank = torch.distributed.get_rank()
-    device = torch.device("cuda", rank % torch.cuda.device_count())
+from torch import distributed as dist
+from diffusers import DiffusionPipeline, ContextParallelConfig
+
+def setup_distributed():
+    if not dist.is_initialized():
+        dist.init_process_group(backend="nccl")
+    rank = dist.get_rank()
+    device = torch.device(f"cuda:{rank}")
     torch.cuda.set_device(device)
-    
-    transformer = AutoModel.from_pretrained("Qwen/Qwen-Image", subfolder="transformer", torch_dtype=torch.bfloat16, parallel_config=ContextParallelConfig(ring_degree=2))
-    pipeline = QwenImagePipeline.from_pretrained("Qwen/Qwen-Image", transformer=transformer, torch_dtype=torch.bfloat16, device_map="cuda")
-    pipeline.transformer.set_attention_backend("flash")
+    return device
+
+def main():
+    device = setup_distributed()
+    world_size = dist.get_world_size()
+
+    pipeline = DiffusionPipeline.from_pretrained(
+        "black-forest-labs/FLUX.1-dev", torch_dtype=torch.bfloat16, device_map=device
+    )
+    pipeline.transformer.set_attention_backend("_native_cudnn")
+
+    cp_config = ContextParallelConfig(ring_degree=world_size)
+    pipeline.transformer.enable_parallelism(config=cp_config)
 
     prompt = """
     cinematic film still of a cat sipping a margarita in a pool in Palm Springs, California
     highly detailed, high budget hollywood movie, cinemascope, moody, epic, gorgeous, film grain
     """
-    
+
     # Must specify generator so all ranks start with same latents (or pass your own)
     generator = torch.Generator().manual_seed(42)
-    image = pipeline(prompt, num_inference_steps=50, generator=generator).images[0]
-    
-    if rank == 0:
-        image.save("output.png")
-
-except Exception as e:
-    print(f"An error occurred: {e}")
-    torch.distributed.breakpoint()
-    raise
-
-finally:
-    if torch.distributed.is_initialized():
-        torch.distributed.destroy_process_group()
+    image = pipeline(
+        prompt,
+        guidance_scale=3.5,
+        num_inference_steps=50,
+        generator=generator,
+    ).images[0]
+
+    if dist.get_rank() == 0:
+        image.save(f"output.png")
+
+    if dist.is_initialized():
+        dist.destroy_process_group()
+
+
+if __name__ == "__main__":
+    main()
 ```
 
+The script above needs to be run with a distributed launcher, such as [torchrun](https://docs.pytorch.org/docs/stable/elastic/run.html), that is compatible with PyTorch. `--nproc-per-node` is set to the number of GPUs available.
+
+/```shell
+`torchrun --nproc-per-node 2 above_script.py`. 
+/```
+
 ### Ulysses Attention
 
 [Ulysses Attention](https://huggingface.co/papers/2309.14509) splits a sequence across GPUs and performs an *all-to-all* communication (every device sends/receives data to every other device). Each GPU ends up with all tokens for only a subset of attention heads. Each GPU computes attention locally on all tokens for its head, then performs another all-to-all to regroup results by tokens for the next layer.
@@ -288,5 +310,26 @@ finally:
 Pass the [`ContextParallelConfig`] to [`~ModelMixin.enable_parallelism`].
 
 ```py
+# Depending on the number of GPUs available.
 pipeline.transformer.enable_parallelism(config=ContextParallelConfig(ulysses_degree=2))
+```
+
+### parallel_config
+
+Pass `parallel_config` during model initialization to enable context parallelism.
+
+```py
+CKPT_ID = "black-forest-labs/FLUX.1-dev"
+
+cp_config = ContextParallelConfig(ring_degree=2)
+transformer = AutoModel.from_pretrained(
+    CKPT_ID, 
+    subfolder="transformer", 
+    torch_dtype=torch.bfloat16, 
+    parallel_config=cp_config
+)
+
+pipeline = DiffusionPipeline.from_pretrained(
+    CKPT_ID, transformer=transformer, torch_dtype=torch.bfloat16,
+).to(device)
 ```