Fix pytorch2onnx failed for interpolate op with PyTorch==1.6.0(mmdet#4646)

[RunningLeon]

Fix issue in mmdet issue 4646

This PR includes:

1. Fix interpolate op's pytorch2onnx conversion error with PyTorch==1.6.0
2. Add unit test of pytorch2onnx for interpolate op

[codecov]

Codecov Report

Merging #848 (ce4cbaa) into master (72e13e6) will increase coverage by 3.36%.
The diff coverage is 27.18%.

@@            Coverage Diff             @@
##           master     #848      +/-   ##
==========================================
+ Coverage   62.97%   66.34%   +3.36%     
==========================================
  Files         145      145              
  Lines        8686     8780      +94     
  Branches     1571     1597      +26     
==========================================
+ Hits         5470     5825     +355     
+ Misses       2951     2641     -310     
- Partials      265      314      +49     

Flag	Coverage Δ
unittests	66.34% <27.18%> (+3.36%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/runner/base_runner.py	67.82% <0.00%> (-1.82%)	⬇️
mmcv/runner/hooks/sampler_seed.py	50.00% <ø> (ø)
mmcv/tensorrt/tensorrt_utils.py	1.91% <3.22%> (+0.87%)	⬆️
mmcv/cnn/bricks/activation.py	87.50% <62.50%> (-12.50%)	⬇️
mmcv/onnx/onnx_utils/symbolic_helper.py	33.72% <72.72%> (+33.72%)	⬆️
mmcv/ops/nms.py	41.17% <75.00%> (+6.73%)	⬆️
mmcv/utils/config.py	89.85% <81.81%> (-0.27%)	⬇️
mmcv/version.py	92.30% <100.00%> (ø)
mmcv/ops/info.py	61.90% <0.00%> (+14.28%)	⬆️
... and 8 more

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[hellock]

Unittests for onnx are not run on Github Actions.

[RunningLeon]

Unittests for onnx are not run on Github Actions.

Hi, @hellock

I believe test_onnx.py would be ran with torch+cpu.
You can click Details of build with cpu to check the results:

tests/test_ops/test_nms.py ....                                          [ 49%]
tests/test_ops/test_nms_rotated.py ss                                    [ 50%]
tests/test_ops/test_onnx.py ss.s..                                       [ 52%]

Check here for the build process.

[hellock]

Seems that test_onnx.py is skipped for build_cuda.