Use Device instead of Backend in TensorIterator (#20690)
Summary:
This PR also moves Device::validate into the header file, which makes
statements like `Device d = kCPU` effectively free.
Device includes the device's index, so TensorIterator::compute_types
now implicitly checks that all CUDA inputs are on the same GPU.
Previously, this was done ad-hoc in places like TensorIterator::binary_op.
Note that zero-dim Tensor (scalars) are NOT required to be on the
same device as other inputs because they behave almost like Python numbers.
TensorIterator handles copying zero-dim Tensors to the common device.
Prior to this PR, TensorIterator would copy zero-dim Tensors between CPU
and GPU, but not between different GPUs (because Backend didn't encode
the GPU index). This removes that restriction.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20690
Differential Revision: D15414826
Pulled By: colesbury
fbshipit-source-id: 1d0ad1f7d663252af36dd4590bcda418c2f7a09f
