Add audio codec models
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# What does this PR do ?

Add NeMo Audio Codec Models to the collection 

**Collection**: [TTS]

# Changelog 
- Add audio codec 16kHz config
- Add NGC model to pretrained list

# Usage

### Automatically load the model from NGC

```python
from  nemo.collections.tts.models  import  AudioCodecModel
nemo_codec_model  =  AudioCodecModel.from_pretrained('audio_codec_16khz_small')
```

### Getting discrete tokens from Audio

```python
import librosa
import torch
audio,sr = librosa.load("<path_to_audio>", sr=16000)
device = 'cuda' if torch.cuda.is_available() else 'cpu'
audio_tensor = torch.from_numpy(audio).unsqueeze(dim=0).to(device)
audio_len = torch.tensor([audio_tensor[0].shape[0]]).to(device)
nemo_codec_model.freeze()
encoded_tokens, encoded_len = nemo_codec_model.encode(audio=audio_tensor, audio_len=audio_len)
```

### Reconstructing audio from discrete tokens

```python
reconstructed_audio = nemo_codec_model.decode(tokens=encoded_tokens, tokens_len=encoded_len)
```

### Listen to audio

```python
import soundfile as sf
output_audio  =  reconstructed_audio.cpu().numpy()[0]
sf.write(<path_to_output_audio>,  output_audio,  samplerate=16000)
```

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- [ ] Did you write any new necessary tests?
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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation
