Do not rescale boxes when their number is 0

[mprzewie]

Before this fix, an attempt to rescale an empty tensor of bounding boxes (i.e. of shape (0,4) ) results in the following error:

RuntimeError: Cannot reshape tensor of 0 elements into shape [0, -1, -4] because the unspecified dimension size -1 can be any value and is ambiguous.

[CLAassistant]

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[codecov]

Codecov Report

Merging #3575 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3575      +/-   ##
==========================================
- Coverage   59.04%   59.00%   -0.04%     
==========================================
  Files         205      205              
  Lines       13731    13731              
  Branches     2315     2315              
==========================================
- Hits         8107     8102       -5     
  Misses       5248     5248              
- Partials      376      381       +5     

Flag	Coverage Δ
#unittests	59.00% <0.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet/models/roi_heads/bbox_heads/bbox_head.py	85.06% <0.00%> (ø)
mmdet/models/dense_heads/rpn_test_mixin.py	77.41% <0.00%> (-6.46%)	⬇️
mmdet/models/roi_heads/test_mixins.py	50.53% <0.00%> (-3.23%)	⬇️

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[hellock]

Thanks for the fix!

[OpenMMLab-Assistant-007]

Hi!
@mprzewie
First of all, we want to express our gratitude for your significant PR in the OpenMMLab project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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