[JAX] grouped_gemm() uses variadic arguments

[huanghua1994]

Description

This PR optimizes the grouped_gemm() implementation in JAX. The original implementation manually flattens all input matrices before lowering to C++ function and manually split the output into a list of tensors. Using variadic arguments allows the code to avoid extra copying of inputs and outputs.

This PR is marked as a draft since PR #1545 breaks both the original implementation and this implementation for MXFP8 on Blackwell. Fixed by PR #1652.

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

Rewrite grouped_gemm() and GroupedGemmPrimitive using variadic arguments.

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

[phu0ngng]

/te-ci jax L0

[phu0ngng]

/te-ci jax L0

[phu0ngng]

Pipeline #26823920 passed.
Ready to merge.