Support convert checkpoint from any directory

[shuheng-liu]

What this does

Convert checkpoint from any directory

How it was tested

Ran convert_checkpoint.sh from another directory.

How to checkout & try? (for the reviewer)

Activate virtual env and run convert_checkpoint.sh from any directory.

Checklist

• I have added Google-style docstrings to important functions and ensured function parameters are typed.
• My PR includes policy-related changes.
  • If the above is checked: I have run the GPU pytests (pytest -m "gpu") and regression tests.

[Copilot]

Pull request overview

This PR updates the checkpoint conversion shell script so it can run the underlying conversion utilities via module execution (python -m ...), avoiding assumptions about the current working directory.

Changes:

• Add a guard to prevent overwriting an existing model.safetensors in the checkpoint directory.
• Switch conversion invocations from relative file paths to python -m opentau.scripts.* module execution.

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