MFSDP v2 Muon: Localize optimizer-step communication within NVLink domains

Follow-up to #5533.

Refine parameter placements during forward and backward so that the Muon optimizer step only needs to communicate within a node or NVLink domain. See http://nv/kOb for more details.

This should reduce cross-node communication in the optimizer path while preserving MFSDP v2’s intended training behavior.

cc @janEbert

Start with follow-up issue #5533 and the linked NVLink-domain details. Trace MFSDP v2 parameter placement through forward and backward, then examine the Muon optimizer step. Done means optimizer-step communication stays within a node or NVLink domain while intended MFSDP v2 training behavior is preserved.

Written by the indexing model from the issue text.