[ONNX] Update len symbolic (#43824)
Summary:
Update len symbolic.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43824
Reviewed By: izdeby
Differential Revision: D23575765
Pulled By: bzinodev
fbshipit-source-id: 0e5c8c8d4a5297f65e2dc43168993350f784c776
