Add XLMRobertaForQuestionAnswering

[sgugger]

One of the missing model task.

[codecov]

Codecov Report

Merging #4855 into master will increase coverage by 2.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4855      +/-   ##
==========================================
+ Coverage   73.93%   76.54%   +2.61%     
==========================================
  Files         128      128              
  Lines       21498    21501       +3     
==========================================
+ Hits        15894    16458     +564     
+ Misses       5604     5043     -561     

Impacted Files	Coverage Δ
src/transformers/__init__.py	99.13% <ø> (ø)
src/transformers/modeling_auto.py	73.60% <ø> (ø)
src/transformers/modeling_xlm_roberta.py	100.00% <100.00%> (ø)
src/transformers/modeling_openai.py	79.58% <0.00%> (-1.39%)	⬇️
src/transformers/modeling_tf_utils.py	87.28% <0.00%> (+0.15%)	⬆️
src/transformers/trainer.py	39.62% <0.00%> (+0.23%)	⬆️
src/transformers/modeling_utils.py	90.26% <0.00%> (+1.42%)	⬆️
src/transformers/modeling_reformer.py	88.19% <0.00%> (+72.36%)	⬆️
src/transformers/configuration_reformer.py	100.00% <0.00%> (+78.94%)	⬆️

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[LysandreJik]

Great, thanks @sgugger!