[ADD] Documentation for data validation and preprocessing

[ravinkohli]

Addresses #319 .
Also, fixes formatting issues which were caused by improper indentation.

[codecov]

Codecov Report

Merging #323 (cca30f7) into development (e493d99) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #323      +/-   ##
===============================================
- Coverage        82.01%   81.98%   -0.04%     
===============================================
  Files              151      152       +1     
  Lines             8652     8669      +17     
  Branches          1322     1325       +3     
===============================================
+ Hits              7096     7107      +11     
- Misses            1088     1094       +6     
  Partials           468      468              

Impacted Files	Coverage Δ
autoPyTorch/api/base_task.py	84.63% <ø> (ø)
autoPyTorch/api/tabular_classification.py	87.50% <ø> (ø)
autoPyTorch/api/tabular_regression.py	96.87% <ø> (ø)
autoPyTorch/datasets/base_dataset.py	80.00% <ø> (ø)
autoPyTorch/datasets/image_dataset.py	31.25% <ø> (ø)
autoPyTorch/datasets/tabular_dataset.py	85.29% <ø> (ø)
autoPyTorch/optimizer/smbo.py	83.19% <ø> (ø)
...ml/traditional_learner/base_traditional_learner.py	94.80% <ø> (ø)
...Torch/pipeline/components/training/metrics/base.py	65.67% <ø> (ø)
autoPyTorch/utils/pipeline.py	72.72% <ø> (ø)
... and 6 more

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[nabenabe0928]

Thanks for the PR.
I put comments except docs/manual.rst .

[nabenabe0928]

docs/manual.rst looks good to me, so I will wait for your commits.

[nabenabe0928]

I think we can delete default?

You can check which file has (xxx), (default=xxx) by find -name "*.py" -exec grep ", (default" {} \; -print | grep "./".

I also found out that some of them have the default value of None while the typing says xxx, but not Optional[xxx].

[ravinkohli]

I think we can delete default?

You can check which file has (xxx), (default=xxx) by find -name "*.py" -exec grep ", (default" {} \; -print | grep "./".

I also found out that some of them have the default value of None while the typing says xxx, but not Optional[xxx].

I have unified the way we document types throughout the project. I have also fixed type information to Optional[type] where previously we had default=None. I think if there is any further change required, lets raise an issue for this and fix it in a different PR.