fix(models): 74 imputer inference mode

[sahahner]

Description

PR from @JesperDramsch from anemoi-model: ecmwf/anemoi-models#75

This resets the nan_locations in the BaseImputer during non-training runs of the code.

Specifically, this enables changes in the dataset after training.

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 #34

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
• I have tested the changes on a single GPU
• I have tested the changes on multiple GPUs / multi-node setups
• I have run the Benchmark Profiler against the old version of the code

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

None