* support custom op `mmcv::cummax` for onnxruntime in mmcv
* fix clang-format lint error
* support mmcv::cummin, reformat codes
* fix merge from master
* add docs for mmcv::cummax and mmcv::cummin
* format doc
* add assertion for torch version, when exporting `cummax` to onnx
* add more comments for torch version
* handle exporting to onnx in `soft_nms`
* commit for test_onnx
* remove `is_in_onnx_export` in softnms
* add more comments
* fix c++ lint error
* add known issues doc for `cummax`
* fix known issues doc