[TTS][refactor] Part 1 - nemo.collections.tts.data

[XuesongYang]

What does this PR do ?

This is a series of refactoring tasks forth.
Task 1: re-organize nemo.collections.tts.data

tts_dataset.py  # move torch/data.py here
g2p_dataset.py  # combine g2p/data/ctc_g2p.py, g2p/data/t5_g2p.py, and g2p/data/heteronym_classification_data.py

# 2. update import paths.

Collection: [Note which collection this PR will affect]

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Additional Information

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[ekmb]

Please check that you have __init__ files in the folders

[XuesongYang]

Please check that you have __init__ files in the folders

This PR did not create new package, neither remove existing init.py. So we are good.