use kernels to support _flash_hub attention backend

[ParagEkbote]

What does this PR do?

As discussed in the issue, this PR adds support for kernels-community/flash-attn kernel. Could you please review?

Fixes #12308

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Who can review?

@sayakpaul

[sayakpaul]

Thanks for this PR. Could you update it with some code examples and results?

[ParagEkbote]

This is the test command, but unable to generate images.

import os
os.environ["DIFFUSERS_ENABLE_HUB_KERNELS"] = "yes"

# Debug: Verify the env var is set
print(f"DIFFUSERS_ENABLE_HUB_KERNELS = {os.environ.get('DIFFUSERS_ENABLE_HUB_KERNELS')}")

import torch
from diffusers import FluxPipeline
from diffusers.quantizers import PipelineQuantizationConfig

# Debug: Check if diffusers sees the env var
from diffusers.models.attention_dispatch import DIFFUSERS_ENABLE_HUB_KERNELS
print(f"Diffusers sees DIFFUSERS_ENABLE_HUB_KERNELS = {DIFFUSERS_ENABLE_HUB_KERNELS}")

# ✅ 3. Load pipeline with quantization
model_id = "black-forest-labs/FLUX.1-dev"
pipe = FluxPipeline.from_pretrained(
    model_id,
    torch_dtype=torch.bfloat16,
    quantization_config=PipelineQuantizationConfig(
        quant_backend="bitsandbytes_4bit",
        quant_kwargs={
            "load_in_4bit": True,
            "bnb_4bit_quant_type": "nf4",
            "bnb_4bit_compute_dtype": torch.bfloat16,
        },
        components_to_quantize=["transformer"],
    ),
).to("cuda")

pipe.transformer.set_attention_backend("_flash_hub")

prompt = "A cat holding a sign that says 'hello world'"
image = pipe(prompt, num_inference_steps=28, guidance_scale=4.0).images[0]
image.save("output.png")

[ParagEkbote]

I'm having issues regarding some of the parameters with the following traceback:

Traceback (most recent call last):
  File "/teamspace/studios/this_studio/diffusers/main.py", line 34, in <module>
    image = pipe(prompt, num_inference_steps=28, guidance_scale=4.0).images[0]
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/torch/utils/_contextlib.py", line 116, in decorate_context
    return func(*args, **kwargs)
  File "/teamspace/studios/this_studio/diffusers/src/diffusers/pipelines/flux/pipeline_flux.py", line 944, in __call__
    noise_pred = self.transformer(
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1751, in _wrapped_call_impl
    return self._call_impl(*args, **kwargs)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1762, in _call_impl
    return forward_call(*args, **kwargs)
  File "/teamspace/studios/this_studio/diffusers/src/diffusers/models/transformers/transformer_flux.py", line 720, in forward
    encoder_hidden_states, hidden_states = block(
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1751, in _wrapped_call_impl
    return self._call_impl(*args, **kwargs)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1762, in _call_impl
    return forward_call(*args, **kwargs)
  File "/teamspace/studios/this_studio/diffusers/src/diffusers/models/transformers/transformer_flux.py", line 443, in forward
    attention_outputs = self.attn(
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1751, in _wrapped_call_impl
    return self._call_impl(*args, **kwargs)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1762, in _call_impl
    return forward_call(*args, **kwargs)
  File "/teamspace/studios/this_studio/diffusers/src/diffusers/models/transformers/transformer_flux.py", line 342, in forward
    return self.processor(self, hidden_states, encoder_hidden_states, attention_mask, image_rotary_emb, **kwargs)
  File "/teamspace/studios/this_studio/diffusers/src/diffusers/models/transformers/transformer_flux.py", line 116, in __call__
    hidden_states = dispatch_attention_fn(
  File "/teamspace/studios/this_studio/diffusers/src/diffusers/models/attention_dispatch.py", line 304, in dispatch_attention_fn
    return backend_fn(**kwargs)
  File "/teamspace/studios/this_studio/diffusers/src/diffusers/models/attention_dispatch.py", line 765, in _flash_attention_hub
    out = flash_attn_func_hub(
TypeError: flash_attn_func() got an unexpected keyword argument 'alibi_slopes'

The same error occurs with dropout_p parameter as well. WDYT?

cc: @sayakpaul