[WIP] FIX: avoid densifying sparse matrix before inverse_transform of OHE

[glemaitre]

closes #493

[pep8speaks]

Hello @glemaitre! Thanks for submitting the PR.

• There are no PEP8 issues in the file imblearn/over_sampling/_smote.py !

[codecov]

Codecov Report

Merging #495 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #495   +/-   ##
=======================================
  Coverage   98.86%   98.86%           
=======================================
  Files          82       82           
  Lines        5020     5020           
=======================================
  Hits         4963     4963           
  Misses         57       57

Impacted Files	Coverage Δ
imblearn/over_sampling/_smote.py	97.24% <100%> (ø)	⬆️

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