Disable BUILD_CAFFE2 from ONNX builds (#90475)
Fixes https://github.com/microsoft/onnx-converters-private/issues/132
@kit1980 and @malfet agreed in disabling ONNX tests for Caffe2 builds.
With this change, exporting models with `operator+export_type=ONNX_ATEN_FALLBACK` will properly test non-caffe2 builds, which is the only scenario for aten fallback after caffe2 deprecation
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90475
Approved by: https://github.com/kit1980, https://github.com/BowenBao
