Feature/optional mlflow

[belerico]

Summary

This PR let MLFlow be an optional dependency. In particular:

• Every agent defines its own log_models method
• Everything related to MLFlow has been moved to sheeprl/utils/mlflow.py
• Added optional MLFlow tests, which can be run with python tests/run_tests_mlflow.py

Type of Change

Please select the one relevant option below:

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

Checklist

Please confirm that the following tasks have been completed:

• I have tested my changes locally and they work as expected. (Please describe the tests you performed.)
• I have added unit tests for my changes, or updated existing tests if necessary.
• I have updated the documentation, if applicable.
• I have installed pre-commit and run locally for my code changes.

Thank you for your contribution! Once you have filled out this template, please ensure that you have assigned the appropriate reviewers and that all tests have passed.