ONNX tests

Directory structure:

.
├── README.md
├── backend.py
├── backend_rep.py
├── backend_test.py
├── gluon_backend_test.py
├── mxnet_backend_test.py
├── mxnet_export_test.py
├── test_cases.py
├── test_models.py
└── test_node.py

• backend.py - MXNetBackend. This file contains prepare().
  This class can be used for both, MXNet and Gluon backend.
• backend_rep.py - MXNetBackendRep and GluonBackendRep for running inference
• backend_test.py - prepare tests by including tests from test_cases.py
• gluon_backend_test.py - Set backend as gluon and execute ONNX tests for ONNX->Gluon import.
• mxnet_backend_test.py - Set backend as gluon and add tests for ONNX->MXNet import/export. Since MXNetBackend for export, tests both import and export, the test list in this file is a union of tests that execute for import and export, export alone, and import alone.
• mxnet_export_test.py - Execute unit tests for testing MXNet export code - this is not specific to any operator.
• test_cases.py - list of test cases for operators/models that are supported for "both", import and export, "import" alone, or "export" alone.
• test_models.py - custom tests for models
• test_node.py - custom tests for operators. These tests are written independent of ONNX tests, in case ONNX doesn't have tests yet or for MXNet specific operators.