Fix to handle null strides in DLPack tensor (#18510)
Summary:
DLPack can have non-strided tensors, which is represented by a nullptr in the place of dl_tensor.strides.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18510
Differential Revision: D14647328
Pulled By: bwasti
fbshipit-source-id: 5364282810a5772cfc2319fc8133fe86fdd84dd1
