refactor: defer decord2 initialization for audio fallback

[alexbowe]

Summary

• Remove the eager decord import from multimodal_utils.
• Load decord2 only when transformers audio loading fails and the audio fallback is actually used.
• Improve the missing-dependency message to point at decord2.
• Add tests for the install hint and fallback behavior.

Validation

• PYTHONPYCACHEPREFIX=/tmp/codex-pycache python3 -m py_compile nemo_rl/data/multimodal_utils.py tests/unit/data/test_multimodal_utils.py
• git diff --check
• uv run pytest tests/unit/data/test_multimodal_utils.py -q (blocked locally: fresh worktree is missing 3rdparty/Gym-workspace/Gym pyproject metadata)

Notes

• This does not make decord2 an optional package; pyproject still lists it as a hard dependency. It just avoids loading the extension at Nemo-RL import time.
• Claude adversarial review found no blockers.

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