[Feature] Add optional Explained Variance logging

[NTR0314]

Description

Adds an optional log_explained_variance flag to PPOLoss (and subclasses) so users can log the critic’s explained‑variance metric during training.

Motivation and Context

The metric helps monitor critic fit and matches sklearn.metrics.explained_variance_score. Also used by other libraries such as stable baselines.

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Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality) <- idk if it is core functionality though.
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

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• My change requires a change to the documentation.
• [] I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/3010

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[vmoens]

LGTM thanks for this!