Add support for exporting onnx split (#15092)
Summary:
* With the update of split output to dynamic list it breaks the export to onnx.
Now split ir becomes two ops: 1. Dynamic[] <= Split(), and 2. out1, out2, out3
<= Prim::ListUnpack. In this fix these two consecutive ops get fused when being
exported to onnx.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15092
Reviewed By: dzhulgakov
Differential Revision: D13583832
Pulled By: houseroad
fbshipit-source-id: 3eb18c871e750921ad6d5cc179254bee9bcf4c99
