Add reference impl for sequence ops (#2380)
Add reference impl for sequence ops (#2380)

* Add reference impl for sequence ops

* add type annotation

* blank lines

Co-authored-by: Emad Barsoum <9473da0d618490b4032b341ffee3076bc4120394@gmail.com>
Co-authored-by: Prasanth Pulavarthi <7b0d53ae08f52c6d3dfa9b25bb9b891a604dc792@microsoft.com>
Co-authored-by: Ke Zhang <368059018edb346f1fd2049a2d55b57da5b85a98@microsoft.com>

onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx