Refactor: SNR & SI_SNR

[Borda]

What does this PR do?

Unify the notation and keep more descriptive functional names
part of #729

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[codecov]

Codecov Report

Merging #712 (1fbff12) into master (fdf5b3f) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #712   +/-   ##
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  Coverage      73%    73%           
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  Files         166    166           
  Lines        6465   6483   +18     
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+ Hits         4730   4748   +18     
  Misses       1735   1735           

[quancs]

The signal noise ratio and signal distortion ratio are a little bit weird.

SNR is often called signal to noise ratio. And SDR is offen called signal to distortion ratio

[Borda]

@PyTorchLightning/core-metrics updated and keeping abbreviations, pls check it now 🐰

[Borda]

@Borda Maybe we can consider to put sisdr and sisnr in one file, because they have very limited difference (or to say sisnr is a special alias for sisdr)

I put sisnr in snr as it seems to be related, right?

Yes, sisnr and snr sounds more related. I'm wondering which is better:
1 put sisnr and snr together;
2 put sisnr and sisdr together;
3 put sisnr, sisdr, snr separately as before

I would go with 1