Fix onnx custom op export & add initial test case (#21321)
Summary:
- Fix typo in ```torch/onnx/utils.py``` when looking up registered custom ops.
- Add a simple test case
1. Register custom op with ```TorchScript``` using ```cpp_extension.load_inline```.
2. Register custom op with ```torch.onnx.symbolic``` using ```register_custom_op_symbolic```.
3. Export model with custom op, and verify with Caffe2 backend.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21321
Differential Revision: D16101097
Pulled By: houseroad
fbshipit-source-id: 084f8b55e230e1cb6e9bd7bd52d7946cefda8e33
