Support vectorized local reduction for p2p-based ReduceScatter overlap

[erhoo82]

Description

Vectorized load/store for p2p-based ReduceScatter overlap.

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

• Vectorize the loads/stores of local reduction after TP ReduceScatter for (1) FP8 comm > FP32 reduce > BF16 out, (2) BF16 comm > FP32 reduce > BF16 out

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