Fix/a2c evaluation

[belerico]

Summary

This PR fixes #192, in particular:

• we have add the evaluation file of the A2C agent cause a call to sheeprl/available_agents.py raises an erorr due the missing evaluation function of the A2C algorithm
• we have also taken into consideration the fact that the evaluation file is manually set to evaluate in the sheeprl/available_agents.py file by saving in the sheeprl.utils.registry.evaluation_registry dictionary the name of the evaluation file where the evaluation funtion resides
• no resolved config.yaml was being saved in the A2C algorithm

Type of Change

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• Bug fix (non-breaking change that solves an issue)

Checklist

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