Add support for testing experimental features

[albertvillanova]

Add support for testing experimental features.

This PR introduces a minimal testing workflow for the new experimental submodule:

• Default behavior unchanged: pytest and make test now exclude experimental tests automatically via addopts = ["-k", "not experimental"] in pyproject.toml
• New target: make test_experimental: runs only the experimental tests
• Directory structure: All tests for features under the trl.experimental namespace should be placed in the directory:
  • tests/experimental/

This setup keeps CI fast and stable (running only the main test suite) while making it easy for contributors to run experimental tests locally (or in a future separate CI job).

Follow-up to:

• 🧪 Add trl.experimental Submodule #4073

[HuggingFaceDocBuilderDev]

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[qgallouedec]

I initially thought that a marker would be more suitable, but the subdirectory solution in test is also good IMO, as it allows the codebases to remain truly separate.

[qgallouedec]

lgtm!