Add TensorBoard logging support

[mooskagh]

Summary

• add a tensorboard_path field to the export config and wire up a TensorBoard logger
• record per-step loss, learning rate, and gradient data plus per-epoch weight statistics during training
• document how to view the exported metrics and add the tensorboardx dependency
• return the training step metrics as a dictionary to keep the API manageable

Testing

• just test-python

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https://chatgpt.com/codex/tasks/task_e_68e96b901ce08331a9254c787cf2f5de