[Fix]: parse scales of list in PyTorch==1.7.0 (#451 in MMPose)

[RunningLeon]

Fix Issue #451 in MMPose: pytorch2onnx failed with PyTorch>=1.7.0 for hrnet_w32_coco_256x192.py. This problem was once metioned in #4221 as well.

This PR includes:

1. update _interpolate_get_scales_if_available to support pytorch2onnx with PyTorch<=1.7.0

[codecov]

Codecov Report

Merging #815 (4a92580) into master (6c57b88) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #815      +/-   ##
==========================================
+ Coverage   62.23%   62.25%   +0.01%     
==========================================
  Files         144      144              
  Lines        8506     8504       -2     
  Branches     1522     1523       +1     
==========================================
  Hits         5294     5294              
+ Misses       2945     2943       -2     
  Partials      267      267              

Flag	Coverage Δ
unittests	62.25% <0.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
mmcv/onnx/onnx_utils/symbolic_helper.py	0.00% <0.00%> (ø)

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