Add Trec dataset

[zhangguanheng66]

The legacy Trec dataset was retired in torchtext.legacy folder. This one yields the raw text strings.

[codecov]

Codecov Report

Merging #1190 (47f3044) into master (ebb7c3a) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1190      +/-   ##
==========================================
+ Coverage   77.93%   78.07%   +0.13%     
==========================================
  Files          65       66       +1     
  Lines        3585     3607      +22     
==========================================
+ Hits         2794     2816      +22     
  Misses        791      791              

Impacted Files	Coverage Δ
torchtext/datasets/__init__.py	100.00% <100.00%> (ø)
torchtext/datasets/trec.py	100.00% <100.00%> (ø)

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