Fix anchor dtype in AnchorGenerator

[fmassa]

Fixes #1335

@Anjum48 can you try this out and let me know if it works for you?

[codecov-io]

Codecov Report

Merging #1341 into master will increase coverage by 0.08%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage    65.5%   65.59%   +0.08%     
==========================================
  Files          75       75              
  Lines        5819     5821       +2     
  Branches      892      892              
==========================================
+ Hits         3812     3818       +6     
+ Misses       1737     1736       -1     
+ Partials      270      267       -3

Impacted Files	Coverage Δ
torchvision/models/detection/roi_heads.py	55.77% <0%> (-0.16%)	⬇️
torchvision/models/detection/rpn.py	79.32% <100%> (+0.09%)	⬆️
torchvision/transforms/transforms.py	80.98% <0%> (+0.98%)	⬆️

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[fmassa]

@Anjum48 this now works with mixed precision.

I tried to minimize the number of casts, but not all tensors are fp16 (as it is mixed precision).