Add shape inference function for Split (#18838)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18838
It turns out that we don't have shape inference function of `Split` op at all. This diff adds that.
Reviewed By: bertmaher
Differential Revision: D14766871
fbshipit-source-id: 535cb4f24bdada603c76579e00e7a39aee93e19f
