[Feature] Add ONNX export support to torch.roll

[grimoire]

Motivation

This PR add ONNX export support to torch.roll.
Related to open-mmlab/mmsegmentation#693 (comment)

Modification

Add torch.roll symbolic file and related onnx test.

[codecov]

Codecov Report

Merging #1194 (8974a71) into master (db580dd) will increase coverage by 0.09%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master    #1194      +/-   ##
==========================================
+ Coverage   67.99%   68.08%   +0.09%     
==========================================
  Files         159      160       +1     
  Lines       10426    10472      +46     
  Branches     1897     1906       +9     
==========================================
+ Hits         7089     7130      +41     
  Misses       2968     2968              
- Partials      369      374       +5     

Flag	Coverage Δ
unittests	68.08% <86.66%> (+0.09%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/onnx/symbolic.py	53.44% <86.66%> (+5.17%)	⬆️
mmcv/utils/parrots_wrapper.py	59.37% <0.00%> (-1.34%)	⬇️
mmcv/utils/trace.py	54.54% <0.00%> (ø)
mmcv/image/photometric.py	99.27% <0.00%> (+<0.01%)	⬆️
mmcv/utils/__init__.py	88.88% <0.00%> (+0.65%)	⬆️
mmcv/ops/cc_attention.py	38.70% <0.00%> (+1.00%)	⬆️
mmcv/onnx/onnx_utils/symbolic_helper.py	40.69% <0.00%> (+1.16%)	⬆️
mmcv/runner/iter_based_runner.py	60.00% <0.00%> (+2.14%)	⬆️

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[RunningLeon]

LGTM after the comments are resolved.

[RunningLeon]

Kindly ping@ZwwWayne