refit clean up and refactoring

[wdykas]

What does this PR do ?

• Refactor of megatron/core/resharding/: dedup across NCCL/Gloo/NVSHMEM (shared base.py), merge
  _plan_multi_dim_lcm + _plan_block_interleaved into _plan_tp
  • Bug fix: _PlanCacheKey now includes src_rank_offset / dst_rank_offset so non-collocated layouts with
    identical sizes don't silently share a plan.
  • Perf: buffer-dtype harmonization + named_refit_tensors + _get_config_tuple cached across refits; MXFP8
    source dequantize prefetched on a side stream; NVSHMEM local copies overlap with the remote pipeline; Gloo
    gains a dedicated copy stream; empty_cache() only when MXFP8 staging happened.
  • Tightened match_local_ops_by_task_id to raise on imbalanced send/recv counts (previously silently dropped
    extras).
  • ~25 new unit tests covering plan cache, copy-service helpers, and non-collocated None-handling; 3
    pre-existing MXFP8 test failures fixed.

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[wdykas]

/ok to test e137aa1

[wdykas]

/ok to test 0b0f089

[wdykas]

/claude review

[claude]

LGTM

[wdykas]

/ok to test 4aa90b9

[wdykas]

/claude review

[claude]

LGTM

[wdykas]

/ok to test 0259fc9

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/25836236302