Introduce Mega-C++ to reduce CPU overhead

[zhongbozhu]

Description

Assistant: GPT5.5 codex

Get rid of CPU overhead whenever CUDA Graph is not applicable. Guarded by NVTE_MEGACPP_GROUPED_LINEAR.

Drop-in replace grouped MLP, ie. FC1 - act - FC2. Target BF16 grouped gemm with cublas grouped gemm backend.

In the future, we can extend to mxfp8 / nvfp4 with cublas backend or even cuteDSL grouped gemm and call cute.jit in C++: NVIDIA/cutlass#3289

Recommend CUDA >= 13.2.1

TODO:

• E2E training with some multimodal THD packing workloads
• Attach before & after screenshots

Fixes # (issue)

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Change A
• Change B

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[zhongbozhu]

/te-ci pytorch L1