Fix/player build agent

[belerico]

Summary

This PR moves the creation of the player agent inside the build_agent method. Furthermore:

• Players are deep-copy of the train agents, but with tied weights
• Every model in the player is setup with a single-device Fabric, i.e. a Fabric object that shares the same precision and accelerator as the main one, but with a single-device strategy to prevent synchronization points during the environment interaction
• Every build_agent method returns the player agent
• Every player agent defines a forward and get_actions methods, plus auxiliary ones

Type of Change

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• 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

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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.
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