Some simple c++ refactoring

[cyyever]

Fix some warnings from clang-tidy due to the lack of parameter reference and using emplace_back to improve performance.

[parmeet]

Thank you @cyyever for making the contribution. Could you please provide a short summary/description of the changes in the description box?

[parmeet]

@cyyever Changes looks good to me..Do you think it would be useful to use emplace_back here as well?

[cyyever]

@cyyever Changes looks good to me..Do you think it would be useful to use emplace_back here as well?

I have added.

[codecov]

Codecov Report

Merging #1327 (35fe716) into master (0d7b9e2) will increase coverage by 2.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1327      +/-   ##
==========================================
+ Coverage   76.43%   78.57%   +2.13%     
==========================================
  Files          67       67              
  Lines        3319     3319              
==========================================
+ Hits         2537     2608      +71     
+ Misses        782      711      -71     

Impacted Files	Coverage Δ
torchtext/utils.py	91.66% <0.00%> (+5.83%)	⬆️
torchtext/vocab/vectors.py	89.69% <0.00%> (+38.78%)	⬆️

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[parmeet]

Thank you @cyyever for your contributions in improving torchtext's C++ code-base :).