Implement Masked Language Modeling (#77)

[bzantium]

• Implement masked language modeling task
• Add hydra config files
• Add test code

[SeanNaren]

Thanks so much @bzantium! the code is very clean as well, once the CI finishes, I'll merge :)

[codecov]

Codecov Report

Merging #173 (b35f154) into master (02c064c) will decrease coverage by 15.42%.
The diff coverage is 82.85%.

@@             Coverage Diff             @@
##           master     #173       +/-   ##
===========================================
- Coverage   84.60%   69.18%   -15.43%     
===========================================
  Files          66       70        +4     
  Lines        1397     1467       +70     
===========================================
- Hits         1182     1015      -167     
- Misses        215      452      +237     

Flag	Coverage Δ
unittests	69.18% <82.85%> (-15.43%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...sformers/task/nlp/masked_language_modeling/data.py	72.97% <72.97%> (ø)
...formers/task/nlp/masked_language_modeling/model.py	91.66% <91.66%> (ø)
...mers/task/nlp/masked_language_modeling/__init__.py	100.00% <100.00%> (ø)
...ormers/task/nlp/masked_language_modeling/config.py	100.00% <100.00%> (ø)
...ning_transformers/task/nlp/summarization/metric.py	27.58% <0.00%> (-72.42%)	⬇️
...transformers/task/nlp/token_classification/data.py	33.87% <0.00%> (-58.07%)	⬇️
...ing_transformers/task/nlp/multiple_choice/model.py	42.42% <0.00%> (-51.52%)	⬇️
...ransformers/task/nlp/token_classification/model.py	51.16% <0.00%> (-48.84%)	⬇️
...ing_transformers/task/nlp/multiple_choice/utils.py	52.38% <0.00%> (-47.62%)	⬇️
...ransformers/task/nlp/translation/datasets/wmt16.py	54.54% <0.00%> (-45.46%)	⬇️
... and 18 more

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[SeanNaren]

Failing test is unrelated, should be fixed separately, so will merge!

[bzantium]

Thank you so much! It's an honour to contribute to this great project :)