Add tanh_normal dist to PPO

[belerico]

Summary

Add tanh-normal distribution for PPO with continuous actions

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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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.