Remove the __main__ restriction from examples

[franchuterivera]

Moving to 'fork' context in the case the number of jobs is equal to 1, and also employing only thread-based computation dask.

[codecov]

Codecov Report

Merging #1053 (2193c53) into development (4b851c4) will increase coverage by 0.19%.
The diff coverage is 94.33%.

@@               Coverage Diff               @@
##           development    #1053      +/-   ##
===============================================
+ Coverage        85.44%   85.64%   +0.19%     
===============================================
  Files              127      129       +2     
  Lines            10177    10323     +146     
===============================================
+ Hits              8696     8841     +145     
- Misses            1481     1482       +1     

Impacted Files	Coverage Δ
autosklearn/util/single_thread_client.py	93.18% <93.18%> (ø)
autosklearn/automl.py	84.42% <100.00%> (-0.28%)	⬇️
autosklearn/ensemble_builder.py	76.61% <100.00%> (+0.04%)	⬆️
autosklearn/smbo.py	81.91% <100.00%> (-0.63%)	⬇️
...ature_preprocessing/select_rates_classification.py	84.50% <0.00%> (-2.82%)	⬇️
..._preprocessing/select_percentile_classification.py	86.20% <0.00%> (-1.73%)	⬇️
.../metalearning/metalearning/kNearestDatasets/kND.py	94.04% <0.00%> (ø)
autosklearn/util/hash.py
... and 4 more

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[franchuterivera]

There seems to be a problem with the mirrors of ubuntu causing non-anaconda runs to fail. Examples are working and I will relaunch the runs tomorrow to gain more confidence (but looks like conda-base runs are ok)

[mfeurer]

Two minor things to go :)

[mfeurer]

Can we check automatically check which other methods are implemented by dask using inspection and automatically add these to the single-threaded client and raise a NotImplementedError so that we don't run into any issues in the future?