fix: create the external metrics tracker only on trainer rank zero

[maocheng23]

Motivation

Stack 2/5 replacing draft #766. Every trainer rank currently constructs its own tracker: W&B/MLflow runs are duplicated world-size times and TensorBoard ranks write the same directory. A 32-rank colocated K3 job would create 32 W&B runs; existing multi-rank disaggregated consumers (the K3 disaggregated recipe runs 4) already suffer this.

Modifications

• _configured_logger creates the external tracker (W&B, MLflow, SwanLab, TensorBoard) on global rank zero only. Every rank keeps the console logger, so console output on multi-rank runs is unchanged; only the duplicate tracker runs/writers go away. Producers are single-process roles and are unaffected.

Related Issues

Splits #766. Stack: #781 teardown-abort ← #2 (this) ← #780 capture-rows ← #783 colocated-core ← #784 hybrid-shard.

Accuracy Test

• test_only_global_rank_zero_creates_an_external_tracker (rank 3 with report_to: wandb gets the console logger and no tracker is created) and test_primary_rank_keeps_the_console_logger_without_a_tracker; the existing tracker-adapter test covers rank 0 creating the tracker.

Checklist

• Format your code according to the Code Formatting with Pre-Commit (black --check and isort --check-only).
• Add unit tests.
• Update documentation as needed (none needed).