Save replicated tensors per-TP-rank and load with file locality

[sbak5]

Save path:

• Assign each TP rank a unique checkpoint key for replicated tensors (._tp_replica{tp_rank}) so each rank writes to its own file.
• Set replica_id to (0, 0, dp_replica_id) for these tensors.

Load path:

• Build FQN→source_rank from checkpoint metadata (distcp filenames).
• In distribute_shards_to_ranks, group shards by source file and assign each group to that rank so each rank reads from its own file when possible, reducing file opens on distributed filesystems.
• Pass checkpoint_dir into apply_loading_parallelization to enable file-locality-aware distribution.

Backward compatibility:

• Detect checkpoints saved without TP-replica postfix keys and strip the postfix from load-side ShardedTensor keys so they match metadata.

What does this PR do ?

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flowchart LR
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    end
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[Phlip79]

We are changing our review process and marking all open, unlabeled PRs as draft. This change will go in effect starting once #3659 is merged.

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