removing dependence on iopath

[parmeet]

This PR remove the dependence on iopath introduced in #1319

This helps resolve issue #1380

[codecov]

Codecov Report

Merging #1381 (a6efcc7) into main (e3799a6) will increase coverage by 2.85%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main    #1381      +/-   ##
==========================================
+ Coverage   76.42%   79.28%   +2.85%     
==========================================
  Files          67       67              
  Lines        3326     3278      -48     
==========================================
+ Hits         2542     2599      +57     
+ Misses        784      679     -105     

Impacted Files	Coverage Δ
torchtext/_download_hooks.py	48.71% <88.88%> (+8.48%)	⬆️
torchtext/utils.py	92.06% <0.00%> (+7.14%)	⬆️
torchtext/vocab/vectors.py	89.69% <0.00%> (+38.78%)	⬆️

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