[TTS] Fix bugs in HiFi-GAN (scheduler, optimizers) and add input_example() in Mixer-TTS/Mixer-TTS-X
# What does this PR do?

* Fixes: 
  * https://github.com/NVIDIA/NeMo/issues/3557 
  * https://github.com/NVIDIA/NeMo/issues/3549
  * Use `self.optimizers` in HiFi-GAN
  * Fix typo in FastPitch's `input_example()`

Signed-off-by: Oktai Tatanov <oktai.tatanov@gmail.com>