Learnable initial recurrent state choice

[belerico]

Summary

In this PR we let the user chose whether or not to learn the initial recurrent state. A fixed initial recurrent state is useful when learning on simple environments, e.g. CartPole-v1

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
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Checklist

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