[ONNX] Fix graph sequence output from loop node (#51305) (#51521)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51521
* Add loop & if node to the list of nodes that could produce sequence type output.
* Switch from `[]` to `at()` to avoid segfault of out of range access.
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D26203112
Pulled By: SplitInfinity
fbshipit-source-id: e990eeed933124b195be0be159271e33fb485063
