Add GSM format support to save function

[mthrok]

We would like to add more format support to "sox_io"'s save function. Full Rate is the codec used in mobile phone system and the libsox torchaudio binds have capability to handle this format.

Steps

Implementation

1. Add GSM to Format enum and get_format_from_string function. The expected string form is "gsm".
2. Add GSM branch in get_save_encoding function. Similar to the case MP3, GSM does not take any encoding or bit depth option. Use SOX_ENCODING_GSM, 16 for return value.
3. Add GSM branch in get_precision function. the return value should be 16.

Update documentation

1. Update the docstring of save function. (add "gsm" in format section then at the bottom before Note: add some explanation from http://sox.sourceforge.net/soxformat.html (do text search "gsm"

Add test

1. Add a new method test_save_gsm to SaveTest class, following other method like test_save_flac. Only parameterize the test_mode. GSM does not support compression/encoding/bits_per_sample.

Building and testing locally

1. Follow the CONTRIBUTING guide to set up development environment.
2. Build and run test with;

BUILD_SOX=1 python setup.py develop && (cd test && pytest torchaudio_unittest/backend/sox_io/save_test.py -v --ff -k gsm )

[prabhat00155]

I can work on this.

[mthrok]

@prabhat00155

Thanks, let me know if you need help.

[imaginary-person]

Hello @prabhat00155, you seem to work at Facebook, so if you haven't started working on it yet, can I please request @mthrok to assign this issue to me, as it has a good first issue label? Thanks!

[imaginary-person]

Hello @mthrok, there don't seem to be any useful details regarding GSM (default bit-depth, etc) at the link you provided. Even if I won't be assigned this issue, this information might be helpful to Prabhat. Thanks!

[mthrok]

@imaginary-person

If you would like to work on something complementary to this issue, you can make info function work for gsm format.
The encoding attribute does not resolve gsm. You can add gsm case here and add test like this one. Bonus point: move the get_encoding function to types.[h|cpp] so that all the encoding/format related functionalities are consolidated to one place.

[imaginary-person]

Thank you, @mthrok. I'll get started right away.