[ONNX] Export split_to_sequence as slice when output number is static (#42744)
Summary:
Optimize exported graph to export slice nodes for aten::split when the number of split outputs are fixed. Previously under some cases these are exported as onnx::SplitToSequence, which is dynamic in tensor output count.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42744
Reviewed By: houseroad
Differential Revision: D23172465
Pulled By: bzinodev
fbshipit-source-id: 11e432b4ac1351f17e48356c16dc46f877fdf7da
