Add NaturalQuestions to datasets

[Cli212]

Description

This PR commits the script for downloading NaturalQuestions from s3 bucket. And also, instructions for using this script is written in README.md.

Additionally, I found that if we want to download triviaqa data, we should use commandpython3 prepare_triviaqa.py --type {rc,unfiltered} rather than python3 prepare_triviaqa.py --version {rc,unfiltered}which README.md shows. So I update this part in README.md too.

Checklist

Essentials

• [x] Changes are complete (i.e. I finished coding on this PR)
• [x] All changes have test coverage

cc @dmlc/gluon-nlp-team

[codecov[bot]]

Codecov Report

Merging #1469 (3298a41) into master (def0d70) will decrease coverage by 0.18%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1469      +/-   ##
==========================================
- Coverage   85.86%   85.67%   -0.19%     
==========================================
  Files          52       52              
  Lines        6911     6920       +9     
==========================================
- Hits         5934     5929       -5     
- Misses        977      991      +14     

Impacted Files	Coverage Δ
src/gluonnlp/data/loading.py	78.11% <0.00%> (-5.29%)	:arrow_down:
src/gluonnlp/op.py	95.71% <0.00%> (+0.63%)	:arrow_up:

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[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1469/add_NQ/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1469/add_NQ/index.html

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1469/add_NQ/index.html