Enabled pipeline fit

[franchuterivera]

addresses #886

[codecov]

Codecov Report

Merging #1096 (426c2e1) into development (8c7154f) will increase coverage by 0.01%.
The diff coverage is 86.84%.

@@               Coverage Diff               @@
##           development    #1096      +/-   ##
===============================================
+ Coverage        85.46%   85.48%   +0.01%     
===============================================
  Files              137      137              
  Lines            10557    10618      +61     
===============================================
+ Hits              9023     9077      +54     
- Misses            1534     1541       +7     

Impacted Files	Coverage Δ
autosklearn/util/pipeline.py	83.58% <72.72%> (+0.76%)	⬆️
autosklearn/automl.py	84.85% <87.34%> (+0.08%)	⬆️
autosklearn/estimators.py	93.47% <90.90%> (+0.40%)	⬆️
autosklearn/smbo.py	84.83% <100.00%> (+0.99%)	⬆️
...ature_preprocessing/select_rates_classification.py	78.87% <0.00%> (-8.46%)	⬇️
autosklearn/evaluation/__init__.py	79.91% <0.00%> (-0.86%)	⬇️
autosklearn/evaluation/train_evaluator.py	73.46% <0.00%> (ø)
autosklearn/util/backend.py	76.51% <0.00%> (+1.77%)	⬆️

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[mfeurer]

Hey, just a bunch of comments on the API. I think we can discuss all of this offline later.

[mfeurer]

Whoa, this feature is way more complicated than I anticipated. Also, do you think #709 can be solved as a side effect?

[mfeurer]

Looks good, I only have four (hopefully small) comment left.

[mfeurer]

Hey, I just tried this locally to debug a weird pipeline and have a few more suggestions. But this features will be really helpful (and already was, uncovered a pretty nasty bug using it)!