[Feature] add onnxruntime test tool

[grimoire]

Hi
This PR adds an ONNXRuntime test tool to align results between PyTorch and ONNXRuntime.
Tested on mattor and restorer models.

[innerlee]

Hi do you want to write a doc on how to use this tool? Add contents to here https://mmediting.readthedocs.io/en/latest/tools_scripts.html?highlight=onnx#convert-to-onnx-experimental .
Particularly, if you give the list of models that have been verified, and post the related command, that would be very great!

[codecov]

Codecov Report

Merging #277 (459862e) into master (ce4ad1f) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
+ Coverage   79.94%   79.99%   +0.05%     
==========================================
  Files         159      160       +1     
  Lines        7942     8003      +61     
  Branches     1177     1185       +8     
==========================================
+ Hits         6349     6402      +53     
- Misses       1449     1456       +7     
- Partials      144      145       +1     

Flag	Coverage Δ
unittests	79.99% <ø> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
mmedit/models/backbones/sr_backbones/__init__.py	100.00% <0.00%> (ø)
mmedit/models/backbones/sr_backbones/rdn.py	86.66% <0.00%> (ø)

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[innerlee]

ort is cryptic. How about renaming the file to onnx_runtime_test.py?

[grimoire]

Close this PR.
I will update the code at #279.