Fix MixerTTS generate_spectrogram types and dimensions
Fixes the typecheck decorator for generate_spectrogram in MixerTTS and the swapping melspec and time dimension (as also done here https://github.com/NVIDIA/NeMo/blob/3687f2eaae36de6dd686caa99d2fe3100a5f4431/nemo/collections/tts/models/mixer_tts.py#L649)

You're probably in the middle of a refactor so sorry if you already got that, @Oktai15  (?)

(just FYI, I just trained MixerTTS from LJ with the current main head and everything worked fine, except this transpose missing when synthesizing and the mismatch of the decorator with the parameters)
