[canary] Refactor: `PromptedAudioToTextLhotseDataset` and `EncDecMultiTaskModel`
Merging to `canary-2` for further development to unblock others -- we should address the rest of comments there.
This PR restores `transformer_bpe_models.py` and `audio_to_text_lhotse.py` to their previous shape and separates out two classes: `EncDecMultiTaskModel` and `PromptedAudioToTextLhotseDataset` that contain the modified code for Canary, generalized to support other model instances of this type.
