[Fix]: Set installation of onnx simplify's related packages to optional (#779)

* Fix onnx, onnxruntime, onnxoptimizer import

* Revert "Fix onnx, onnxruntime, onnxoptimizer import"

This reverts commit 820e3785a1144eb85e1ba22688dada681ea4c829.

* remove simplify from init file

* update test for onnx simplify

* update onnx doc

docs/onnx.md

@@ -20,6 +20,14 @@ register_extra_symbolics(opset_version)
 
 `mmcv.onnx.simplify` is based on [onnx-simplifier](https://github.com/daquexian/onnx-simplifier), which is a useful tool to make exported ONNX models slimmer by performing a series of optimization. However, for Pytorch models with custom op from `mmcv`, it would break down. Thus, custom ops for ONNX Runtime should be registered.
 
+### Prerequisite
+
+`mmcv.onnx.simplify` has three dependencies: `onnx`, `onnxoptimizer`, `onnxruntime`. After installation of `mmcv`, you have to install them manually using pip.
+
+```bash
+pip install onnx onnxoptimizer onnxruntime
+```
+
 ### Usage
 
 ```python
@@ -27,7 +35,8 @@ import onnx
 import numpy as np
 
 import mmcv
-from mmcv.onnx import simplify
+from mmcv.onnx.simplify import simplify
+
 dummy_input = np.random.randn(1, 3, 224, 224).astype(np.float32)
 input = {'input':dummy_input}
 input_file = 'sample.onnx'

mmcv/onnx/__init__.py

 from .info import is_custom_op_loaded
-from .simplify import simplify
 from .symbolic import register_extra_symbolics
 
-__all__ = ['register_extra_symbolics', 'simplify', 'is_custom_op_loaded']
+__all__ = ['register_extra_symbolics', 'is_custom_op_loaded']

tests/test_ops/test_onnx.py

@@ -271,7 +271,7 @@ def test_roipool():
 
 
 def test_simplify():
-    from mmcv.onnx import simplify
+    from mmcv.onnx.simplify import simplify
 
     # only support PyTorch >= 1.5.0
     if version.parse(torch.__version__) < version.parse('1.5.0'):