Validate PackedAttention token offsets (#33330)
This pull request introduces validation for the `token_offset` input in
the CUDA implementation of the PackedAttention operator, specifically
for the unfused backend. The main goal is to ensure that invalid
`token_offset` values are detected early and reported with a clear error
message, improving robustness and debuggability. The change includes
both the implementation of the validation logic and new tests to verify
its correctness.
**Validation improvements for PackedAttention:**
* Added a CUDA kernel and a host function
(`ValidatePackedAttentionTokenOffset`) to check that all `token_offset`
values are within the valid range `[0, B * S)` for the unfused backend,
and integrated this validation into
`PackedAttention<T>::ComputeInternal`.
[[1]](diffhunk://#diff-4e4c1980e908e7c273f52041c3dc123cb7a8c4ee20779fb28b4975911556c9dbR31-R84)
[[2]](diffhunk://#diff-450dced704ee2fc03ee3611224808eb8a00671f7587e48599ce17c21d5e7c9e8R16-R21)
[[3]](diffhunk://#diff-fd7731431d1e80901e297b2ce7f1018fc7305faf58145cb36733bd19b0aeb9d2R270-R279)
* Updated relevant CUDA kernels to handle out-of-range `token_offset`
values gracefully by skipping computation or zeroing out invalid output
regions.
[[1]](diffhunk://#diff-4e4c1980e908e7c273f52041c3dc123cb7a8c4ee20779fb28b4975911556c9dbR115-R117)
[[2]](diffhunk://#diff-4e4c1980e908e7c273f52041c3dc123cb7a8c4ee20779fb28b4975911556c9dbR371-R374)
**Testing enhancements:**
* Added a new unit test (`InvalidTokenOffset_Unfused_CUDA`) to verify
that invalid `token_offset` values for the unfused backend are correctly
detected and result in a descriptive error message.
