[PyTorch] Enable MXFP8 LayerNorm and RMSNorm

[timmoon10]

Description

MXFP8 LayerNorm and RMSNorm were disabled in #1457 to avoid failures with certain tensor dimensions. This PR restores support when the tensor dimensions are multiples of 128 and cuDNN norm backend is enabled (by setting NVTE_NORM_FWD_USE_CUDNN=1 in the environment).

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

• Enable MXFP8 LayerNorm and RMSNorm

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