Import ONNX model zoo programs to iree_tests

[ScottTodd]

https://github.com/onnx/models, https://onnx.ai/models/

The iree_tests/onnx/ test suite currently imports "node" tests from onnx/backend/test/case/node. There are also a few "model" tests there onnx/backend/test/case/model, but many more models exist at https://github.com/onnx/models.

Import process:

• Fork https://github.com/nod-ai/SHARK-TestSuite/blob/main/iree_tests/onnx/import_tests.py
• For each model in the model zoo:
  • Determine some test inputs
  • Run through onnxruntime (?)
  • Record test outputs
  • Generate a test case in iree_tests
• Add test config files like https://github.com/nod-ai/SHARK-TestSuite/blob/main/iree_tests/configs/onnx_cpu_llvm_sync.json, going through import -> compile -> run for all test cases
• Prune the set of imported models as needed (e.g. dedup different opset versions)