Update ONNX Flatten to accept negative indices in opset 11 (#30751)
Summary:
Update ONNX Flatten to accept negative indices in opset 11.
With this change, some cases of flatten do not rely on the input rank being available.
Fixes : https://github.com/pytorch/pytorch/issues/30512 .
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30751
Reviewed By: hl475
Differential Revision: D18946904
Pulled By: houseroad
fbshipit-source-id: a6fa30a9182fff92211e505a19325525c6112f19
