Add Aliases to Model Cards
 - Adds new `aliases` property to PretrainedModelInfo
 - Allow models to use `from_pretrained()` with aliases

Example:
```python
PretrainedModelInfo(
    pretrained_model_name="tts_en_tacotron2",
    location="https://api.ngc.nvidia.com/...nemo",
    description="model description",
    class_=cls,
    aliases=["Tacotron2-22050Hz"],
)
model.from_pretrained("tts_en_tacotron2")
model.from_pretrained("Tacotron2-22050Hz")  # Both will load the same model
```