[BugFix] Change ppo mujoco example to match paper results

[albertbou92]

Description

Minor adjustments to the mujoco ppo example to make it more stable and match the paper results.

Motivation and Context

Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

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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)
• 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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• I have read the CONTRIBUTION guide (required)
• 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/1714

• 📄 Preview Python docs built from this PR

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✅ You can merge normally! (4 Unrelated Failures)

As of commit 2361765 with merge base c2edf35 ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
• Unit-tests on Linux CPU / tests (3.9) / linux-job (gh)

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Habitat Tests on Linux / tests (3.9, 11.6) / linux-job (gh)

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[vmoens]

LGTM thanks!