refactor: use TensorDict NestedKey directly

[MohammedThowfiq]

Replace the internal key compatibility alias with tensordict.utils.NestedKey throughout library signatures and tests. Remove the alias from _types.py and update the file-structure reference to describe the remaining helpers. Key validation and composition behavior are unchanged.

Fixes #132.

Validation on Python 3.12.13, TensorDict 0.14.0 and PyTorch 2.13.0:

• All configured pre-commit checks passed, including Ruff checks and formatting.
• 537 focused tests passed across key helpers, contexts, modules, hooks, descriptions, workflows, latent methods and attribution.
• No references to the removed alias remain in source or tests; git diff --check passed.

The full notebook/integration suite was not run.

[cubic-dev-ai]

No issues found across 19 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Auto-approved: Refactor swaps the internal UnraveledKey alias for TensorDict's NestedKey across type signatures, imports, and tests, and removes the now-unused alias definition. No behavioral change is introduced; it is a bounded, clearly beneficial cleanup backed by comprehensive tests.

Re-trigger cubic</sub>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.45%. Comparing base (09d5dd3) to head (df9aa61).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #134   +/-   ##
=======================================
  Coverage   98.45%   98.45%           
=======================================
  Files          54       54           
  Lines        4336     4340    +4     
=======================================
+ Hits         4269     4273    +4     
  Misses         67       67           

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