Fix RoIAlign and RoIPool for non-contiguous gradients

[fmassa]

Fixes a bug discovered by @lopuhin in #826 (comment)

[codecov-io]

Codecov Report

Merging #920 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #920   +/-   ##
=======================================
  Coverage   60.65%   60.65%           
=======================================
  Files          60       60           
  Lines        4926     4926           
  Branches      733      733           
=======================================
  Hits         2988     2988           
  Misses       1750     1750           
  Partials      188      188

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[lopuhin]

Thank you so much @fmassa 👍