[ckpt, model] refactor: remove unused helper functions

[yaoyu-33]

Summary

• Remove unused is_tensor_parallel() from models/conversion/model_bridge.py.
• Remove unused private _transpose_first_dim() from training/checkpointing.py.
• Both helpers had no repo callers in src/megatron/bridge, tests, examples, docs, or scripts.

Usage evidence

• git grep -n "is_tensor_parallel" -- src/megatron/bridge tests examples docs scripts "*.py" "*.md" only found the removed definition.
• git grep -n "_transpose_first_dim\|num_splits_first" -- src/megatron/bridge tests examples docs scripts "*.py" "*.md" only found the removed private helper.
• is_tensor_parallel() was not exported from models/__init__.py or models/conversion/__init__.py.

Validation

• uv run --no-sync python -m py_compile src/megatron/bridge/models/conversion/model_bridge.py src/megatron/bridge/training/checkpointing.py
• uv run --no-project --with ruff ruff check src/megatron/bridge/models/conversion/model_bridge.py src/megatron/bridge/training/checkpointing.py
• uv run --no-project --with ruff ruff format --check src/megatron/bridge/models/conversion/model_bridge.py src/megatron/bridge/training/checkpointing.py
• git diff --check origin/main...HEAD
• uv run pre-commit run --all-files is blocked locally because uv cannot remove <local uv cache>/git-v0/db/f679917ede501d2d while resolving transformer-engine @ git+https://github.com/NVIDIA/TransformerEngine.git@release_v2.15 (Permission denied).

Unit tests were not run locally per dispatcher policy. This cleanup removes unreachable/uncalled helpers only.

[copy-pr-bot]

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[yaoyu-33]

/ok to test f838e09

[yaoyu-33]

Usage evidence for cleanup:

• is_tensor_parallel() had no repo callers: git grep -n "is_tensor_parallel" -- src/megatron/bridge tests examples docs scripts "*.py" "*.md" found only the removed definition in model_bridge.py.
• _transpose_first_dim() had no repo callers: git grep -n "_transpose_first_dim\|num_splits_first" -- src/megatron/bridge tests examples docs scripts "*.py" "*.md" found only the removed private helper in checkpointing.py.
• is_tensor_parallel() was not exported from models/__init__.py or models/conversion/__init__.py; _transpose_first_dim() was private and undocumented.
• Focused static validation passed; full pre-commit is blocked locally by a permission error in the uv cache while resolving TransformerEngine.

[claude]

LGTM — clean removal of two unused helpers (is_tensor_parallel and _transpose_first_dim). Verified zero callers across src/, tests/, examples/, scripts/, and docs/. Neither function was exported from any __init__.py.

Minor note: the PR body references a local path (/home/yuya/.cache/...) in the validation section — not in code, so no action needed, but worth redacting for hygiene.

Suggested test cases

No perf tests impacted.

[yaoyu-33]

/ok to test e059f59