Upload source distribution to PyPi

[AbdBarho]

What does this PR do?

Upload source distribution to PyPi. Continuation of #588

I am unsure if this is strictly necessary since we basically upload wheels for every supported platform, but it won't hurt ¯\_(ツ)_/¯

Related #533

I have considered consoldating the env from the last 2 steps to the global env, but I don't want any steps (especially external actions) to access the token.

Example sdist:
https://test.pypi.org/project/formers/0.0.16rc379/#files

[danthe3rd]

This looks great - thanks a lot! Is it ready to merge?
Btw, we are limited to 10GB of storage on pypi. One version takes ~300MB, so we can "afford" ~30 versions.
I suggest we keep the last ~10 RC, and last ~20 tagged versions - I'm working on a script to automate the cleaning of the previous RC.

[AbdBarho]

it is ready to merge!

The answer to your suggestion depends entirely on how often do you plan on releasing new versions. My biggest concern is old versions of the library not being available anymore.

~20 tagged versions + release once a month = ~1.5 years average lifetime of a version, which doesn't sound like much.

Maybe after 1.5 years the project is big enough to request another storage increase from pypi :p

This is totally up to you and maybe others on the team to decide, I don't really mind either way.

[danthe3rd]

Yeah, our versions will also quickly be outdated, as they rely on a specific PyTorch version. We could keep the source distribution tho for tagged releases