[Feature] add onnx simplify

[RunningLeon]

This PR adds mmcv.onnx.simplify module, which is based on onnx-simplifier, an useful tool to simplify and optimize ONNX models.
Because we want to simplify models with MMCV's custom ops, which should be registered before simplifying. Thus, we have to modify source codes from onnx-simplifier, and include them in mmcv repository.
This PR includes:

1. mmcv.onnx.simplify module;
2. Simple unit test;
3. Simple doc;

After this PR merged, pytorch2onnx.py in many projects such as mmdetection could be updated.

[codecov]

Codecov Report

Merging #751 (5b1aca8) into master (741e52d) will decrease coverage by 2.09%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #751      +/-   ##
==========================================
- Coverage   64.77%   62.67%   -2.10%     
==========================================
  Files         136      139       +3     
  Lines        7999     8266     +267     
  Branches     1412     1484      +72     
==========================================
  Hits         5181     5181              
- Misses       2549     2816     +267     
  Partials      269      269              

Flag	Coverage Δ
unittests	62.67% <0.00%> (-2.10%)	⬇️

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Impacted Files	Coverage Δ
mmcv/onnx/__init__.py	0.00% <0.00%> (ø)
mmcv/onnx/simplify/__init__.py	0.00% <0.00%> (ø)
mmcv/onnx/simplify/common.py	0.00% <0.00%> (ø)
mmcv/onnx/simplify/core.py	0.00% <0.00%> (ø)

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[daquexian]

Hi @RunningLeon , thanks for your excellent work! I'm the maintainer of onnx-simplifier (and onnx optimizer). Will it help if I support custom op registration in onnx-simplifier? mmcv will benefit from the future update of onnx-simplifier if it directly uses onnx-simplifier package.

[RunningLeon]

Hi, @daquexian,
Very glad to hear from you. Totally agree with you! It would definitely benefit a lot if onnx-simplifier supports customer ops registration in the future.
Thanks.