[TTS] Fix bugs in VocoderDataset and TTSDataset
**Collection**: [TTS]

# Changelog 
- Fix bug with random sampling and `numpy` to `torch` converting in `__getitem__`
- Fix bug with `phoneme_probability` field for `EnglishCharsTokenizer`
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation