Add mel codec checkpoints
# What does this PR do ?

This PR adds pointers to Mel Codec checkpoints.

**Collection**: TTS

# Changelog 
- Added checkpoints to class
- Added checkpoints to docs

# Usage


```python
# automatically load model
from nemo.collections.tts.models import AudioCodecModel
nemo_codec_model = AudioCodecModel.from_pretrained('mel_codec_22khz_medium')

# get discrete tokens from audio
import librosa
import torch
audio, _ = librosa.load(path_to_input_audio, sr=nemo_codec_model.sample_rate)

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)

# reconstruct audio from tokens
reconstructed_audio, _ = nemo_codec_model.decode(tokens=encoded_tokens, tokens_len=encoded_len)

# save reconstructed audio
import soundfile as sf
output_audio = reconstructed_audio.cpu().numpy().squeeze()
sf.write(path_to_output_audio, output_audio, nemo_codec_model.sample_rate)
```

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# Additional Information
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