Don't switch executors mid test (#63830)

Don't switch executors mid test (#63830)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63830

It's really not safe to change the executor out from under models that may have
already been partially compiled.
ghstack-source-id: 136526228

Test Plan:
```
DEBUG=1 CFLAGS="-fsanitize=address" CXXFLAGS="-fsanitize=address" USE_LLVM=$(realpath ../llvm-project/install) CMAKE_PREFIX_PATH=$CONDA_PREFIX python setup.py install
LD_PRELOAD=/lib64/libasan.so.5 numactl -C3 pytest -v --cov --cov-report xml:test/coverage.xml --cov-append onnx/test_pytorch_onnx_onnxruntime.py::TestONNXRuntime_opset11 -s
```

Reviewed By: desertfire

Differential Revision: D30504489

fbshipit-source-id: 188581cb53f0cf5bd3442d1e9d46e8c0c7e124f8