[Fix] Fix bbox clamp in pt1.10

[RangiLyu]

Motivation

FCOS and AutoAssign cannot run with PyTorch 1.10.
Error message:

RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation: [torch.cuda.FloatTensor [2, 4, 20, 20]], which is output 0 of ReluBackward0, is at version 1; expected version 0 instead.

After using with torch.autograd.set_detect_anomaly(True): to debug and find out that it was caused by the ReLU in bbox branch.

Modification

Replace F.relu(bbox_pred) with bbox_pred.clamp(min=0)

[codecov]

Codecov Report

Merging #7074 (9ae7878) into dev (4b87ddc) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##              dev    #7074      +/-   ##
==========================================
- Coverage   62.39%   62.38%   -0.01%     
==========================================
  Files         329      329              
  Lines       26176    26175       -1     
  Branches     4432     4432              
==========================================
- Hits        16332    16330       -2     
- Misses       8974     8975       +1     
  Partials      870      870              

Flag	Coverage Δ
unittests	62.36% <50.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/models/dense_heads/fcos_head.py	76.12% <0.00%> (-0.16%)	⬇️
mmdet/models/dense_heads/autoassign_head.py	94.02% <100.00%> (ø)
mmdet/core/bbox/samplers/random_sampler.py	75.00% <0.00%> (-5.56%)	⬇️
mmdet/apis/train.py	15.66% <0.00%> (ø)
mmdet/core/bbox/samplers/sampling_result.py	74.60% <0.00%> (+1.58%)	⬆️

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[ZwwWayne]

Several things to be done after merging this PR:

1. Should add comments above the line of code to indicate the reason for the change.
2. CI of PyTorch 1.10.