Allow using C10_DECLARE_TENSOR_TYPE and C10_DEFINE_TENSOR_TYPE from any namespace (#18158)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18158
They didn't work when called from other namespaces before because they didn't fully specify the c10 namespace.
Reviewed By: ezyang
Differential Revision: D14512187
fbshipit-source-id: a496b89a1bbe2b56137cfae03ab94a60f38d7068
