[TTS][refactor] Part 3 - nemo.collections.tts.g2p.models

[XuesongYang]

What does this PR do ?

Task 3: break down nemo.collections.tts.g2p.models and save to nemo.collection.tts.models

• base.py # move and merge tts/g2p/models/g2p_model.py::G2PModel class here because it is the base model for g2p.
• g2p_ctc.py # move and rename tts/g2p/models/ctc_g2p.py here. File name uses the same format as fastpitch_*.py
• g2p_t5.py # move and rename tts/g2p/models/t5_g2p.py here.
• g2p_heteronym_classification.py # move and rename tts/g2p/models/heteronym_classifiation.py here.

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[redoctopus]

LGTM.

Brief ramble: I'm trying but can't think of a better solution than renaming heteronym_classification -> g2p_heteronym_classification because it might be slightly misleading (only does the classification and not full G2P). But it falls enough under that umbrella that I can't complain too much, and makes for better organization of files in TTS anyway.

[XuesongYang]

renaming heteronym_classification -> g2p_heteronym_classification

yeah, same feeling as yours though. The reason why I added the prefix g2p is because HeteronymClassificationModel is used to find the best pronunciation from candidate dict entries. So literally, it is part of converting graphemes into phonemes.

heteronym_classification itself sounds more like a different task, like purely text classification irrelevant to any pronunciations.