Make Hifigan jittable
This PR makes changes to the HifiGan module that should make it jittable/torchscriptable. nemo export should work as well.

Verified compatible with checkpoint on NGC.

```python
from nemo.collections.tts.models import HifiGanModel
model = HifiGanModel.from_pretrained(model_name="tts_hifigan")
model.export("test.onnx")
```

Signed-off-by: Ryan Leary <rleary@nvidia.com>