Fix ATen Fallback for BUILD_CAFFE2=0 for ONNX-only ops (#88504)
Follow-up for #87735
Once again, because BUILD_CAFFE2=0 is not tested for ONNX exporter, one scenario slipped through. A use case where the model can be exported without aten fallback when operator_export_type=ONNX_ATEN_FALLBACK and BUILD_CAFFE2=0
A new unit test has been added, but it won't prevent regressions if BUILD_CAFFE2=0 is not executed on CI again
Fixes #87313
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88504
Approved by: https://github.com/justinchuby, https://github.com/BowenBao
