Handle truncation properly with PPO

[vwxyzjn]

Description

This PR is a prototype for #198 and sail-sg/envpool#194

Types of changes

• Bug fix
• New feature
• New algorithm
• Documentation

Checklist:

• I've read the CONTRIBUTION guide (required).
• I have ensured pre-commit run --all-files passes (required).
• I have updated the documentation and previewed the changes via mkdocs serve.
• I have updated the tests accordingly (if applicable).

If you are adding new algorithms or your change could result in performance difference, you may need to (re-)run tracked experiments. See #137 as an example PR.

• I have contacted vwxyzjn to obtain access to the openrlbenchmark W&B team (required).
• I have tracked applicable experiments in openrlbenchmark/cleanrl with --capture-video flag toggled on (required).
• I have added additional documentation and previewed the changes via mkdocs serve.
  • I have explained note-worthy implementation details.
  • I have explained the logged metrics.
  • I have added links to the original paper and related papers (if applicable).
  • I have added links to the PR related to the algorithm.
  • I have created a table comparing my results against those from reputable sources (i.e., the original paper or other reference implementation).
  • I have added the learning curves (in PNG format with width=500 and height=300).
  • I have added links to the tracked experiments.
  • I have updated the overview sections at the docs and the repo
• I have updated the tests accordingly (if applicable).

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Name	Status	Preview	Updated
cleanrl	✅ Ready (Inspect)	Visit Preview	Nov 12, 2022 at 1:55AM (UTC)

[vwxyzjn]

Due to the lack of performance improvement, temporarily closing this PR.

[KaleabTessera]

Shouldn't this use terminal_observation to handle trunc correctly?