[Enhancement] DatasetLoader for BERT pre-training

[eric-haibin-lin]

This PR introduces a DatasetLoader that launches a worker pool to prefetch datasets based on urls.
The main thread creates a new dataloader whenever the current dataset is exhausted.
The feature is currently in the script folder.

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Codecov Report

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[codecov]

Codecov Report

Merging #799 into master will increase coverage by <.01%.
The diff coverage is 52.63%.

@@            Coverage Diff             @@
##           master     #799      +/-   ##
==========================================
+ Coverage   90.38%   90.39%   +<.01%     
==========================================
  Files          66       65       -1     
  Lines        6378     6280      -98     
==========================================
- Hits         5765     5677      -88     
+ Misses        613      603      -10

Impacted Files	Coverage Δ
src/gluonnlp/utils/files.py	45.09% <100%> (+8.73%)	⬆️
src/gluonnlp/data/stream.py	85.56% <18.18%> (-4.06%)	⬇️
src/gluonnlp/data/dataloader.py

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