Multi text img columns

[jrzaurin]

1. Added the possibility of using multiple text or image columns
2. Added option to fuse the model via a ModelFuser
3. Added multi-target losses
4. Fixed some bugs

[codecov]

Codecov Report

Attention: Patch coverage is 88.82175% with 74 lines in your changes missing coverage. Please review.

Project coverage is 94.38%. Comparing base (90c8193) to head (ae3b84f).
Report is 54 commits behind head on master.

Files	Patch %	Lines
...dedeep/load_from_folder/image/image_from_folder.py	66.66%	13 Missing ⚠️
pytorch_widedeep/losses_multitarget.py	92.59%	12 Missing ⚠️
pytorch_widedeep/models/model_fusion.py	89.28%	12 Missing ⚠️
pytorch_widedeep/models/wide_deep.py	79.66%	12 Missing ⚠️
pytorch_widedeep/training/_wd_dataset.py	80.00%	5 Missing ⚠️
...ytorch_widedeep/training/_multiple_lr_scheduler.py	33.33%	4 Missing ⚠️
pytorch_widedeep/callbacks.py	94.44%	3 Missing ⚠️
...widedeep/load_from_folder/text/text_from_folder.py	80.00%	3 Missing ⚠️
pytorch_widedeep/models/text/rnns/basic_rnn.py	50.00%	3 Missing ⚠️
...ep/load_from_folder/tabular/tabular_from_folder.py	83.33%	2 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
- Coverage   95.01%   94.38%   -0.63%     
==========================================
  Files         109      117       +8     
  Lines        6615     7445     +830     
==========================================
+ Hits         6285     7027     +742     
- Misses        330      418      +88     

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