feat(models): adding leaky boundings

[pinnstorm]

Description

Adding leaky bounding equivalents to bounding.py for use with model variables. Potential fix for AIFS Single "Rain Pox" issue. May lead to better back-propagation of gradients when adding bounding as a final layer in the model.

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

This issue was motivated by the "rain pox" seen in the AIFS Single in production where we get grid points predicting unphysical high rainfall in isolation from any other weather systems. Adding LeakyHardTanh bounding to soil moisture (instead of HardTanh) is a potential fix for this issue.

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
• If the new feature introduces modifications at the config level, I have made sure to update Pydantic Schemas and default configs accordingly

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

[anaprietonem]

Thanks for the changes Ewan! LGTM