E2E TTS fixes
Several fixes to get the FastSpeech2-HiFiGAN training to work (#3360). The normal training command should work now.

- Config reference to n_mels corrected
- Fix output type issue when `splices` is None for FS2-HiFiGAN
- Removed `spec` from calls to forward function in FS2-HiFiGAN
- Changed type of FS2-HiFiGAN output to AudioSignal
- Added keywords to functions that require kwargs due to typing
- `precessor` -> `preprocessor` typo fixes
- FastSpeech2 duration loss type fixed, model output type changed to reflect reality

Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>