Fix Pydantic validation error for Vertex Tensorboard args

[hsuan-lun-chiang]

Description

When running MaxText benchmarks, use_vertex_tensorboard and vertex_tensorboard_project may be converted to None. The MaxTextConfig Pydantic model currently enforces strict string types, causing a ValidationError (Input should be a valid string).

This change updates the config definition to Optional[str]. This allows the fields to accept None during validation.

Error Log

Tests

python3 -m benchmarks.benchmark_runner on-device --base_output_directory gs://maxtext-test/ --run_name="test-run" --num_steps=5

Logs

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[NicoGrande]

LGTM

[richjames0]

lgtm