QuestionAnsweringTask and QuestionAnsweringData with SQuADDatSource.

[karthikrangasai]

What does this PR do?

Fixes #577
Fixes #540
Fixes #541

• Update / rewrite QuestionAnsweringTask and QuestionAnsweringData to handle HF AutoModelForQuestionAnswering as backbone.
• Provide SQuAD Dataset as a SQuADDataSource.
• Add examples and documentation for both the Task and Data classes.
• Added tests (needs some work here).

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• Did you write any new necessary tests? [not needed for typos/docs]
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[codecov]

Codecov Report

Merging #607 (28f282e) into master (c512c31) will decrease coverage by 0.65%.
The diff coverage is 76.75%.

@@            Coverage Diff             @@
##           master     #607      +/-   ##
==========================================
- Coverage   90.01%   89.36%   -0.66%     
==========================================
  Files         195      197       +2     
  Lines        9975    10502     +527     
==========================================
+ Hits         8979     9385     +406     
- Misses        996     1117     +121     

Flag	Coverage Δ
unittests	89.36% <76.75%> (-0.66%)	⬇️

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

Impacted Files	Coverage Δ
flash/__main__.py	95.23% <ø> (ø)
flash/text/seq2seq/__init__.py	100.00% <ø> (ø)
flash/text/question_answering/finetuning.py	19.78% <19.78%> (ø)
flash/text/question_answering/data.py	86.17% <86.17%> (ø)
flash/text/question_answering/model.py	90.57% <90.57%> (ø)
flash/text/__init__.py	100.00% <100.00%> (ø)
flash/text/question_answering/__init__.py	100.00% <100.00%> (ø)
flash/text/question_answering/cli.py	100.00% <100.00%> (ø)
flash/text/seq2seq/core/model.py	75.32% <0.00%> (-1.30%)	⬇️
... and 6 more

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[ethanwharris]

Awesome work @karthikrangasai just a few small comments

[ethanwharris]

Really awesome work!!!! Just a few small comments, maybe just remove the deserializer / serve stuff for now and we can add it in a follow-up PR with an example?

[karthikrangasai]

Sure, I will remove the serve and deserializer code.