Hybrid ASR-TTS models
# What does this PR do ?

This PR adds support for hybrid ASR-TTS models.

**Features:**
- [x] hybrid ASR-TTS model (`ASRWithTTSModel`) from CTC-BPE and RNNT-BPE for finetuning
- [x] hybrid ASR-TTS model (`ASRWithTTSModel`) from ASR model config for training from scratch
- [x] fusing BatchNorm to improve the quality of the finetuned models
- [x] text-to-text datasets
- [x] mixing ASR and text-to-text datasets
- [x] enhancer support: will be added after https://github.com/NVIDIA/NeMo/pull/5565

Dependent on:
- https://github.com/NVIDIA/NeMo/pull/5671
- https://github.com/NVIDIA/NeMo/pull/5774

**Collection**: [ASR]

# Changelog 
- **new hybrid ASR-TTS model** `ASRWithTTSModel` in `nemo/collections/asr/models/hybrid_asr_tts_models.py`
- **text-to-text datasets**: `TextToTextDataset` (map-style) + `TextToTextIterableDataset` (iterable) in `nemo/collections/asr/data/text_to_text.py`
- **fused BatchNorm**: `nemo/collections/asr/parts/submodules/batchnorm.py` + support for fused BatchNorm in ConformerEncoder in `nemo/collections/asr/parts/submodules/conformer_modules.py`
- **default config** for hybrid model: `examples/asr/conf/asr_tts/hybrid_asr_tts.yaml`
- **example scripts** for finetuning existing models and training from scratch: `examples/asr/asr_with_tts/speech_to_text_bpe_with_text_finetune.py` + `examples/asr/asr_with_tts/speech_to_text_bpe_with_text.py`

# Usage
- `examples/asr/asr_with_tts/speech_to_text_bpe_with_text_finetune.py` for finetuning pretrained ASR model with TTS
- `examples/asr/asr_with_tts/speech_to_text_bpe_with_text.py` for training ASR with TTS from scratch

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [x] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

