refactor(train): relocate to tools/training.py for bridge compatibility

[MdSadiqMd]

Description

Relocate train() + HookedTransformerTrainConfig from transformer_lens/train.py to transformer_lens/tools/training.py, typed against nn.Module so TransformerBridge trains through it.

Motivation: train.py:17 hard-imports HookedTransformer and is re-exported from __init__.py:10, so at 4.0 the package import breaks unless the loop leaves the HT namespace.

Changes:

• transformer_lens/tools/training.py: New home for TrainConfig (renamed from HookedTransformerTrainConfig) and train(), typed against nn.Module
• transformer_lens/train.py: Deprecation shim with DeprecationWarning on use
• transformer_lens/tools/__init__.py: Add training to lazy submodules
• tests/integration/model_bridge/test_native_training.py: Add test_train_loop_trains_bridge_via_train_config, fix InductionDataset to return single samples

Acceptance:

• tools/training.py exists, no HookedTransformer import
• TransformerBridge (via boot_native) trains through train()
• from transformer_lens import train still imports, emits DeprecationWarning, delegates
• Tests under tests/integration/model_bridge/test_native_training.py
• make unit-test + uv run mypy . clean

Fixes #1589

Type of change

• New feature (non-breaking change which adds functionality)

Screenshots

N/A

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility