[MXFP8/FP4-param-gather] Post processing after forced param AG in eval

[WanZzzzzz]

What does this PR do ?

Fixes FP8/MXFP8/FP4 parameter-gather convergence when eval runs between training steps.

When eval disables forward pre-hooks, start_param_sync(force_sync=True) can complete parameter all-gather synchronously. In that path there is no async gather handle for the next train forward hook to wait on (line), so the usual post-all-gather processing was skipped in next train step. In eval, the post processing is also skipped if the job enables overlap_param_gather (line). With FP8 (blockwise scaling) or FP4 param gather, this can leave columnwise weight storage stale while rowwise storage has been refreshed, causing the next train backward to use staled weights. With MXFP8 param+reuse_grad_buff_for_mxfp8_param_ag, the AG'ed weights will not be copied(quantized) from param buffer to model weights, which is not correct.

In this PR we add post processing in bucket group's start_param_sync for synchronous gather, hence making sure the above cases are correctly handled.

Validated the fix on DSv3 proxy model pretraining for FP4 param gather:

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[WanZzzzzz]

/ok to test da05c93

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

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