unify AudioToMelSpectrogramPreprocessor and AudiotoSpectrogram
        this is very close to 
https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/asr/modules/audio_preprocessing.py#L95. it would just be adding one or two args to return intermediate STFT from AudioToMelSpectrogramPreprocessor or sub-classing it.

_Originally posted by @krishnacpuvvada in https://github.com/NVIDIA/NeMo/pull/5356#discussion_r1018418772_
      