change base.yml path from MaxText to maxtext

[susanbao]

Description

Move the base.yml path from src/MaxText/configs to src/maxtext/configs/post_train. In the AI-Hypercomputer/maxtext#3044, maxText move all config files from src/MaxText/configs to src/maxtext/configs. We need to modify the base.yml path for this.

XLML DAGs failed for using maxtext/configs/base.yml, error:

FileNotFoundError: [Errno 2] No such file or directory: '/deps/src/MaxText/configs/base.yml'

Tests

No need to test.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run one-shot tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.