Add support for config flag with no file path

[dipannita08]

Description

Currently users need to pass in the file path to config file when running MaxText workloads (src/maxtext/configs/post_train/rl.yml)

This change adds support to resolve or infer the config without requiring the user to specify the config path, while previous functionality is still supported.

FIXES: b/491604657

Tests

• CI
• Run a workload with config flag:

python3 -m maxtext.trainers.pre_train.train
  run_name=$RUN_NAME \
  base_output_directory=$OUTPUT_PATH \
  dataset_type=synthetic \
  steps=$STEPS

• Run a workload with full config file path:

python3 -m maxtext.trainers.pre_train.train src/maxtext/configs/base.yml \
  run_name=$RUN_NAME \
  base_output_directory=$OUTPUT_PATH \
  dataset_type=synthetic \
  steps=$STEPS

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[bvandermoon]

Thank you @dipannita08. Is it significantly harder to infer the config needed based on the command (like you had mentioned previously) and just pass that directly through?