[Dev] [Draft] FP8 params support for megatron-fsdp

[kunlunl]

What does this PR do ?

Major changes:

• Make Megatron-FSDP’s AG pipeline support using different data parallel buffers, because MXFP8 has different quantization direction in forward and backward passes.
• Decouple the FP8-related logic from the main workflow and provide a unified abstraction to 1) operate the raw data storage of different recipes; 2) Create or discard transpose cache for different recipes.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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Pre-checks

• I want this PR in a versioned release and have added the appropriate Milestone (e.g., Core 0.8)
• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
• I have run the autoformatter.sh on my PR

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[shjwudp]

Please resolve the conflicts and make sure all tests pass.

I think naming the file that handles mixed precision as mixed_precision.py would be more appropriate.

[shjwudp]

Tested on Llama3-8B. Convergence was stable across all configurations.

Performance Result:

• M-FSDP-mxfp8 achieved a 34% speedup over M-FSDP-bf16 (1385 TFLOPs vs. 1033 TFLOPs).
• M-FSDP-mxfp8 reduced memory usage by 21.93 GB (155.89 GB vs. 177.82 GB for bf16).

Full test report: https://api.wandb.ai/links/nvidia/z3nax4om