[TTS] bugfix IPAG2P and refactor to remove duplicate process.
Signed-off-by: Xuesong Yang <1646669+XuesongYang@users.noreply.github.com>

# What does this PR do ?

1. fixed a bug for IPAG2P: the returned self.phoneme_dict mismatches between the one obtained from [self.parse_as_cmu_dict](https://github.com/NVIDIA/NeMo/blob/44de3950f0bf2f3e85d7df1a962eb24674f4fa4e/nemo_text_processing/g2p/modules.py#L320) and the one directly updated from a [dict obj](https://github.com/NVIDIA/NeMo/blob/44de3950f0bf2f3e85d7df1a962eb24674f4fa4e/nemo_text_processing/g2p/modules.py#L351). The former will return something like `{..., "WIRE": [["ˈ", "w", "a", "ɪ", "ɚ"], ["ˈ", "w", "a", "ɪ", "ɹ"]], ...}`, while the latter will return like `{..., "WIRE": ["ˈ", "w", "a", "ɪ", "ɚ"], ...}`.
2. refactored to remove duplicate processes for the case when the phoneme dict input is a dict object and for the case when the phoneme dict is a file.
3. added type hints and docstrings for the whole class `IPAG2P`.

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

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