Pytorch2onnx (#3075)

* Update pytorch2onnx.py which using new logic to convert pytorch to ONNX

* use standard API to check whether in ONNX convert process

* only compare the score value while verifying results between ONNX and pytorch

* move import onnx before import torch, or something weird will happen

* use real images for input

* modifying the way of calling nms

* modify docstring for bbox2result, and remove unnecessary part for onnx exporting

* modify the 'Convert to ONNX' part in docs

* replace or to | in docstring

* update according to the latest mmcv

* add normalize part

* raise error while using low version mmcv

* minor update

* minor update

Co-authored-by: Jiarui XU <xvjiarui0826@gmail.com>