add SP support for `flash_varlen_hub` backend (#13479)
* add mask support for flash backend
* fix test case
* refactor test
* add protection
* fix comment
* update according to suggestion
* revert change
* fix according to claude review
* add test converage for QwenImage
* add SP support and fix non-contiguous mask for flash_varlen kernel
* revert change
* Update tests/models/testing_utils/parallelism.py
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
* Update tests/models/testing_utils/parallelism.py
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
* drop `_padded_to_unpad`
* follow `if _parallel_config is None` pattern
* rename `attn_mask_2d`
* move check to the top
* make comment clear
* move non-contiguous-attention-mask as default dummy data
* revert and update
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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
