Fix `max_length_q` and `max_length_k` types to `flash_attn_varlen_func`
Author: HollowMan6
Pull request: yes

Also add notes asking users to set `TORCHDYNAMO_CAPTURE_SCALAR_OUTPUTS=1` or call `torch._dynamo.config.capture_scalar_outputs = True`, as currently this will cause a graph break.

# What does this PR do?

Fix `max_length_q` and `max_length_k` types to `flash_attn_varlen_func`. Also add notes asking users to set `TORCHDYNAMO_CAPTURE_SCALAR_OUTPUTS=1` or call `torch._dynamo.config.capture_scalar_outputs = True`, as currently this will cause a graph break.

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Fixes #35588


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