Rename batchsize to batchsize_per_gpu

[sophie-xhonneux]

Description

Rename the batchsize config value to be more self explanatory. This will break configs from past checkpoints unfortunately.

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

Issue Number

Closes #497

Code Compatibility

• I have performed a self-review of my code

Code Performance and Testing

• I ran the uv run train and (if necessary) uv run evaluate on a least one GPU node and it works
• If the new feature introduces modifications at the config level, I have made sure to have notified the other software developers through Mattermost and updated the paths in the $WEATHER_GENERATOR_PRIVATE directory

Dependencies

• I have ensured that the code is still pip-installable after the changes and runs
• I have tested that new dependencies themselves are pip-installable.
• I have not introduced new dependencies in the inference portion of the pipeline

Documentation

• My code follows the style guidelines of this project
• I have updated the documentation and docstrings to reflect the changes
• I have added comments to my code, particularly in hard-to-understand areas

Additional Notes

[clessig]

Can you please ruff it. Then it's ready to be merged.

[clessig]

Did a quick test and looks all good.