Naming conventions in torchaudio models

[jimchen90]

🚀 Feature

For size names, add naming conventions n_* in torchaudio models

Motivation

As mentioned in torchaudio conventions, "For size names, the prefix n_ is used (e.g. "a tensor of size (n_freq, n_mel)") whereas dimension names do not have this prefix (e.g. "a tensor of dimension (channel, time)")"

How about we change the size names in torchaudio models wav2letter and waveRNN(#705, #724 ) so that they follow this n_* format?

For example: num_* to n_*

[vincentqb]

Good idea. Since wav2letter has not been part of a release yet, we can go ahead and modify the interface without being BC-breaking.