MXFP8 training bug fixes for quantized_model_init and Torch FSDP fp8 all gather

[sudhu2k]

Description

Ensure keep_fp8_weight_transpose_cache flag is set to True not only for autocast but also for quantized_model_init.
Fix padding during fp8 all-gather

Fixes: #15425
#15420

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

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

[alextmagro]

LGTM! Just sync with Matthias on that one padding thing please.