Enable group mode (varlen) kernel generation for PyTorch integration

[chinmaydk99]

Proposed changes

This PR enables group mode (variable-length attention) kernel generation for PyTorch's CK SDPA backend.

Checklist

Please put an x into the boxes that apply. You can also fill these out after creating the PR. If you're not sure, please don't hesitate to ask.

• I have added tests relevant to the introduced functionality, and the unit tests are passing locally
• I have added the test to REGRESSION_TESTS list defined at the top of CMakeLists.txt in tests/CMakeLists.txt, IF the test takes more than 30 seconds to run.
• I have added inline documentation which enables the maintainers with understanding the motivation
• I have removed the stale documentation which is no longer relevant after this pull request
• (If this change is user-facing) I have added release notes which provide the end users with a brief summary of the improvement from this pull request
• I have run clang-format on all changed files
• Any dependent changes have been merged

Discussion

The change is minimal (single line deletion) but enables a significant feature: variable-length attention support for ROCm users via PyTorch's torch.nn.attention.varlen API.

[poyenc]

May I know why the group-mode kernels were not included previously, and why they are required now?