Shorten Multi30k code by generating docstring

[cpuhrsch]

Reduces the code by about 60 lines by adding the option of including docstring to the header decorator

[codecov]

Codecov Report

Merging #1185 (970edfb) into master (3e05f58) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1185      +/-   ##
==========================================
+ Coverage   77.70%   77.88%   +0.17%     
==========================================
  Files          65       65              
  Lines        3571     3585      +14     
==========================================
+ Hits         2775     2792      +17     
+ Misses        796      793       -3     

Impacted Files	Coverage Δ
torchtext/datasets/common.py	88.76% <100.00%> (+4.24%)	⬆️
torchtext/datasets/multi30k.py	96.42% <100.00%> (+0.68%)	⬆️

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