[TTS][zh] refine hardcoded lowercase for ASCII letters.
# What does this PR do ?

1. relaxed the hardcoded lowercase for non-Chinese characters. This PR provided customization for tokenization/g2p by introducing `ascii_letter_prefix` and `ascii_letter_case` arguments. These flexible arguments helped to discriminate Chinese phonemes from ascii letters. 
2. avoided to set up cases during the initialization of tokenizer because G2P should take good care of cases, mostly because tokenizer is instantiated after g2p according to [`def _setup_tokenizer()`](https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/tts/models/fastpitch.py#L231).
3. Refined docstring for g2p and tokenizer.
4. Changes will not break pre-trained NGC models because all nightly tests are passed.
```
$ pytest --nightly tests/collections/tts/models
===================================================================================== 27 passed, 3 warnings in 330.07s (0:05:30) =====================================================================================
```

**Collection**: [Note which collection this PR will affect]

# Changelog 
- Add specific line by line info of high level changes in this PR.

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```python
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# Additional Information
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