Cache device on TensorImpl; clean up TensorImpl constructors. (#18833)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18833
ghimport-source-id: 6f2be25fcc5e6be3ffe20582e604bd2c1fbab66b
Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18833 [STACK] Cache device on TensorImpl; clean up TensorImpl constructors.**
* #18832 [STACK] Disallow changing the device of a tensor via set_.
* #18831 [STACK] Stop swapping in Storages of the wrong device for Tensors.
1) We cache device on TensorImpl. This means we can access the device without a virtual function and allows us to more easily extend TensorImpls (because they don't need to figure out how to store the Device for themselves).
2) Clean up TensorImpl APIs. We had a constructor that took a TensorTypeId and an allocator and would allocate a Storage based on the recognized types of TensorTypeIds. Instead, we just have two different constructors: one for types with a storage, one without.
Reviewed By: dzhulgakov
Differential Revision: D14766230
fbshipit-source-id: 745b8db84dcd6cb58f1a8675ad3ff8d033bc50df
