Fix 3d attention mask broadcasting in MHA (#27464)
(1) Fix 3d attention mask broadcasting in MHA
(2) Refactor attention python tests of LLM (add MHA)
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This pull request includes a minor fix to the attention mask
broadcasting logic in the CUDA attention kernel, as well as the addition
of a missing license header in a test file.
Improvements to attention mask broadcasting logic:
* Updated the logic in `attention.cc` to clarify and correct how
broadcasting is determined for 3D attention masks, ensuring the batch
dimension always broadcasts and the heads dimension broadcasts only if
its size is 1. This improves correctness and clarity for different mask
shapes.
Documentation and compliance:
* Added the standard Microsoft MIT license header to the
`test_onnx_attention/__init__.py` file to ensure proper licensing
information is included.
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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
