Set default dataset_type to synthetic

[aireenmei]

Description

set default from tfds -> synthetic since we are deprecating the tfds pipeline

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[gemini-code-assist]

Code Review

This pull request updates the default dataset type from 'tfds' to 'synthetic' across the configuration files (base.yml, types.py) and the checkpointing module (checkpointing.py). There are no review comments, so I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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