[PyTorch] Add stub NestedTensor_is_contiguous function (#73997)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73997
Allows development of code that needs to fail gracefully if/when non-contiguous NestedTensors are added.
ghstack-source-id: 150983355
Test Plan: CI
Reviewed By: zrphercule
Differential Revision: D34647900
fbshipit-source-id: 711ed4d375f33a817159a6e87e3c9cea56566b01
(cherry picked from commit ebaac86f605735e09d294f7f8074e3d22c27f080)
