Add Cityscapes DataModule + clean up DM docs

[InCogNiTo124]

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes #135.

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[codecov]

Codecov Report

Merging #136 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
- Coverage   91.32%   91.30%   -0.03%     
==========================================
  Files          86       84       -2     
  Lines        4174     4049     -125     
==========================================
- Hits         3812     3697     -115     
+ Misses        362      352      -10     

Flag	Coverage Δ
#unittests	91.30% <ø> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
pl_bolts/datamodules/sklearn_datamodule.py

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[InCogNiTo124]

Due to technical reasons (uncorrelated with this PR) I'm unable to test this, can somebody please try it out?

[williamFalcon]

Awesome.

1. Can you add to the docs.
2. Can you add the generator argument to the random_split function?

[Borda]

cool, just add Docs and source reference

[mergify]

This pull request is now in conflict... :(

[williamFalcon]

@InCogNiTo124 awesome PR! Looking forward to more datamodules :)