Narrow exception handling and add logging for torchcodec fallback

[Copilot]

Addresses review feedback on exception handling when importing torchcodec.decoders.AudioDecoder. The broad Exception catch was silently swallowing unrelated errors (internal bugs, environment issues) and making failures harder to diagnose.

Changes:

• Catch specific exceptions (ImportError, OSError, RuntimeError) instead of broad Exception
• Log warning with exception details when falling back to torchaudio-based decoder

try:
    from torchcodec.decoders import AudioDecoder
except (ImportError, OSError, RuntimeError) as e:
    logger.warning("Failed to import torchcodec AudioDecoder, falling back to torchaudio: %s", e)
    AudioDecoder = CompatAudioDecoder

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