[Feature] Add KL approximation in PPO loss metadata

[albertbou92]

Description

This PR adds KL approximation in PPO loss metadata, which provides an idea of the distance between the policy that collected the data and the current policy.

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/2166

• 📄 Preview Python docs built from this PR

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❌ 5 New Failures, 8 Unrelated Failures

As of commit 973d641 with merge base eaa3dd8 ():

NEW FAILURES - The following jobs have failed:

• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration
• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration
• Examples Tests on Linux / tests (3.9, 12.1) / linux-job (gh)
  RuntimeError: Command docker exec -t bb657c5f584e3f581e7c4641dbc5c798f4c2279fa33767ea28c5ac7a79406cf2 /exec failed with exit code 1
• Habitat Tests on Linux / tests (3.9, 12.1) / linux-job (gh)
  RuntimeError: Command docker exec -t 7cc15156538f6df0592734bf2a58906256773678360189b3be6231089e80fa3b /exec failed with exit code 139
• Unit-tests on Linux / tests-optdeps (3.10, 12.1) / linux-job (gh)
  RuntimeError: Command docker exec -t 343c26119d0e6567e74c78985d2bc391bb1195e9d897fc5fec3833dc03cbf048 /exec failed with exit code 1

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

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

• Unit-tests on Linux / tests-cpu (3.10) / linux-job (gh) (trunk failure)
  test/test_env.py::TestInfoDict::test_auto_register[device0]
• Unit-tests on Linux / tests-cpu (3.11) / linux-job (gh) (trunk failure)
  test/test_env.py::TestInfoDict::test_auto_register[device0]
• Unit-tests on Linux / tests-cpu (3.8) / linux-job (gh) (trunk failure)
  test/test_env.py::TestInfoDict::test_auto_register[device0]
• Unit-tests on Linux / tests-cpu (3.9) / linux-job (gh) (trunk failure)
  test/test_env.py::TestInfoDict::test_auto_register[device0]
• Unit-tests on Linux / tests-gpu (3.10, 12.1) / linux-job (gh) (trunk failure)
  test/test_transforms.py::TestVC1::test_vc1_spec_against_real[False-device0]
• Unit-tests on Linux / tests-olddeps (3.8, 11.6) / linux-job (gh) (trunk failure)
  test/test_transforms.py::TestVecNorm::test_state_dict_vecnorm
• Unit-tests on Linux / tests-stable-gpu (3.10, 11.8) / linux-job (gh) (trunk failure)
  test/test_transforms.py::TestVC1::test_vc1_spec_against_real[False-device0]
• Unit-tests on Windows / unittests-cpu / windows-job (gh) (trunk failure)
  test/test_collector.py::TestLibThreading::test_auto_num_threads

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[vmoens]

LGTM thanks!