[TTS] update waveglow export
# What does this PR do ?

Closes #3509 on the NeMo side. Requires a fix in Riva as well.

**Collection**: TTS

# Changelog 
- There was a name collusion with audio that resulted in a onnx dynamic axes error
- input_example was changed to be compliant with the new spec of (return dict,) as opposed to the older return dict
  
**PR Type**:
- [x] Bugfix