[ONNX] Fix flatten operator (#45632)
Summary:
Even when dim is None, there are cases when flatten can be exported.
Also enable test_densenet in scripting mode
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45632
Reviewed By: VitalyFedyunin
Differential Revision: D24116994
Pulled By: bzinodev
fbshipit-source-id: 76da6c073ddf79bba64397fd56b592de850034c4
