feat: Allow empty config path to run from overrides

[gmertes]

Description

Allows the user to pass no config to run, couple and retrieve to run completely from the overrides, useful for quick debugging.

Example:
anemoi-inference run input=mars checkpoint=mycheckpoint.ckpt

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Code Compatibility

• I have performed a self-review of my code

Code Performance and Testing

• I have added tests that prove my fix is effective or that my feature works
• I ran the complete Pytest test suite locally, and they pass

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📚 Documentation preview 📚: https://anemoi-inference--206.org.readthedocs.build/en/206/

[HCookie]

Looks good to me,
Can you add some comments to the docs to show this usage?

[gmertes]

Done