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

scripts/onnx/test.sh

@@ -24,6 +24,11 @@ done
 set -- "${UNKNOWN[@]}" # leave UNKNOWN
 
 pip install pytest scipy hypothesis
+# JIT C++ extensions require ninja.
+pip install ninja --user
+# ninja is installed in /var/lib/jenkins/.local/bin
+export PATH="/var/lib/jenkins/.local/bin:$PATH"
+
 if [[ $PARALLEL == 1 ]]; then
     pip install pytest-xdist
 fi
@@ -49,11 +54,14 @@ pytest "${args[@]}" \
   -k \
   'not (TestOperators and test_full_like) and not (TestOperators and test_zeros_like) and not (TestOperators and test_ones_like) and not (TestModels and test_vgg16) and not (TestModels and test_vgg16_bn) and not (TestModels and test_vgg19) and not (TestModels and test_vgg19_bn)' \
   --ignore "$top_dir/test/onnx/test_pytorch_onnx_onnxruntime.py" \
+  --ignore "$top_dir/test/onnx/test_custom_ops.py" \
   "${test_paths[@]}"
 
 # onnxruntime only support py3
+# "Python.h" not found in py2, needed by TorchScript custom op compilation.
 if [[ "$BUILD_ENVIRONMENT" == *py3* ]]; then
   pip install --user onnxruntime
   pytest "${args[@]}" "$top_dir/test/onnx/test_pytorch_onnx_onnxruntime.py"
+  pytest "${args[@]}" "$top_dir/test/onnx/test_custom_ops.py"
 fi