[Test] Add tests for Tree

[kurtamohler]

Stack from ghstack (oldest at bottom):

• -> [Test] Add tests for Tree #2738

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2738

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❌ 6 New Failures, 8 Unrelated Failures

As of commit 9a9272b with merge base dbc8e2e ():

NEW FAILURES - The following jobs have failed:

• Build Windows Wheels / pytorch/rl / upload / wheel-py3_9-cpu (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_rl__3.9_cpu_
• Build Windows Wheels / pytorch/rl / upload / wheel-py3_9-cuda11_8 (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_rl__3.9_cu118_
• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
  Process completed with exit code 1.
• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
  Process completed with exit code 1.
• Unit-tests on Linux / tests-olddeps (3.8, 11.6) / linux-job (gh)
  RuntimeError: Command docker exec -t 28d96ba4f1d32051193cf654d0dede03a70851ff6181c0283462475a669c6246 /exec failed with exit code 1
• Unit-tests on Windows / unittests-cpu / windows-job (gh)
  Process completed 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

• Build Windows Wheels / pytorch/rl / upload / wheel-py3_9-cuda12_4 (gh) (trunk failure)
  Unable to download artifact(s): Artifact not found for name: pytorch_rl__3.9_cu124_
• Build Windows Wheels / pytorch/rl / upload / wheel-py3_9-cuda12_6 (gh) (trunk failure)
• Habitat Tests on Linux / tests (3.9, 12.4) / linux-job (gh) (trunk failure)
  AttributeError: _ARRAY_API not found
• Unit-tests on Linux / tests-optdeps (3.11, 12.4) / linux-job (gh) (trunk failure)
  test/test_transforms.py::TestTokenizer::test_transform_inverse
• Wheels / build-wheel-windows (3.10, 3.10.3) (gh) (trunk failure)
  This request has been automatically failed because it uses a deprecated version of actions/upload-artifact: v3. Learn more: https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/
• Wheels / build-wheel-windows (3.11, 3.11) (gh) (trunk failure)
  This request has been automatically failed because it uses a deprecated version of actions/upload-artifact: v3. Learn more: https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/
• Wheels / build-wheel-windows (3.12, 3.12) (gh) (trunk failure)
• Wheels / build-wheel-windows (3.9, 3.9) (gh) (trunk failure)
  This request has been automatically failed because it uses a deprecated version of actions/upload-artifact: v3. Learn more: https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/

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[kurtamohler]

I mostly wrote this to become more familiar with Tree. Maybe we don't strictly need these tests since a lot of it is also tested in TestMCTSForest, but I figured it might be good to have some tests that are independent of the MCTSForest API. If you think we don't need this, I'm fine with just closing it