Add lazy::Shape::numel() (#68314)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68314
Add a convenience to lazy::Shape for counting the number of elements (by multiplying out the dimensions). This is a method on Tensor, and in switching other lazy tensor shape utils to use aten shape inference, we need numel counts.
Test Plan: add unit tests
Reviewed By: alanwaketan
Differential Revision: D32409138
fbshipit-source-id: 3ae725300f8826d38e45412f46501d5e5f776fb2
