[MXFP8]Update param buffer before AG in eval

[WanZzzzzz]

What does this PR do ?

Since we do a forced-sync param AG in eval, we need to copy the main param to param buffer before AG to ensure we gather the updated weights.

A related fix in Megatron-LM (NVIDIA/Megatron-LM#4563): Also in eval, after param AG, we need to copy the values in param buffer back to model weights (param.data) to update the weights. This is required for mxfp8+fp8-param-gather+reuse-grad-buff-for-mxfp8-param-ag. If we do not do this, the eval step will use the staled weights( those from last iteration).

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• Add specific line by line info of high level changes in this PR.

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[Phlip79]

/ok to test 1f1353f

[yaoyu-33]

@WanZzzzzz feels like it could use some tests to make sure the behvior is correct

[WanZzzzzz]

@yaoyu-33 @gautham-kollu
I added coverage in this MCore PR instead of a MBridge recipe test because the behavior being fixed is in MCore post-AG processing path. This fix in Mbridge just ensures the MCore code path will work correctly. Also I looked at the existing MBridge recipe tests, and they don’t currently expose the DDP knobs needed to exercise this bug, specifically overlap_param_gather, fp8_param_gather/fp4_param_gather, and reuse_grad_buf_for_mxfp8_param_ag in a train -> eval -> train flow.

The new MCore tests explicitly simulate train -> eval -> train with overlap_param_gather enabled and compare both train and eval loss against the non-param-gather reference:

MXFP8: tests/unit_tests/test_fp8_param.py::TestFP8Param::test_mxfp8_eval_transition
NVFP4: tests/unit_tests/test_fp4_param.py::TestFP4Param::test_nvfp4_eval_transition

[gautham-kollu]

/ok to test f71fe72

[cuichenx]

training code coverage is tested in mcore

[WanZzzzzz]

Test code is here:

NVIDIA/Megatron-LM#4563