Change dtype from torch.float32 to torch.int32 in anchor_utils.py

[zhiqwang]

Following the upstream pytorch/vision#4409 .

[codecov]

Codecov Report

Merging #166 (087d040) into master (992b8c3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #166   +/-   ##
=======================================
  Coverage   96.25%   96.25%           
=======================================
  Files          11       11           
  Lines         561      561           
=======================================
  Hits          540      540           
  Misses         21       21           

Flag	Coverage Δ
unittests	96.25% <ø> (ø)

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[zhiqwang]

cc @dkloving . I remember that you also encountered some issues here when you did the fp16 conversion in #107 before, if this modification affects your subsequent use, please don't mind letting me know.