Fix/evaluate no grad

[belerico]

Summary

This PR fixes a problem related to the increasing memory consuption for dreamer-v3 during the evaluation phase. In particular it let the user choose whether to disable gradients during the evaluation phase with the config parameter disable_grads, i.e. one can launch an evaluation with:

python sheeprl_eval.py checkpoint_path=/path/to/checkpoint.ckpt disable_grads=True

Type of Change

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• New feature (non-breaking change that adds functionality)

Checklist

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• 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.

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