Auto-convert GPU arrays that support the __cuda_array_interface__ protocol (#20584)
Summary:
This PR implements auto-conversion of GPU arrays that support the `__cuda_array_interface__` protocol (fixes #15601).
If an object exposes the `__cuda_array_interface__` attribute, `touch.as_tensor()` and `touch.tensor()` will use the exposed device memory.
#### Zero-copy
When using `touch.as_tensor(...,device=D)` where `D` is the same device as the one used in `__cuda_array_interface__`.
#### Implicit copy
When using `touch.as_tensor(...,device=D)` where `D` is the CPU or another non-CUDA device.
#### Explicit copy
When using `torch.tensor()`.
#### Exception
When using `touch.as_tensor(...,device=D)` where `D` is a CUDA device not used in `__cuda_array_interface__`.
#### Lifetime
`torch.as_tensor(obj)` tensor grabs a reference to `obj` so that the lifetime of `obj` exceeds the tensor
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20584
Differential Revision: D15435610
Pulled By: ezyang
fbshipit-source-id: c423776ba2f2c073b902e0a0ce272d54e9005286
