Update NeMo core for melgan
Split off updates to nemo core that are required for #1418 

Changes:
- Remove empty *_step() methods from ModelPT
- Treat None inside of ElementType's parameters as a Void that matches anything
  - Use case being removing the default 16kHz sample_rate from AudioSignal and replacing it with None. This allows a module with type AudioSignal() to match ANY AudioSignal regardless of sample_rate

Minor Changes:
- Add Singleton NeuralType
- Update wrap_training_step such it is doesn't require a dictionary to be passed
- Other cosmetic changes