Add Tensor.is_cpu (#78887)
Fixes #76872
Not sure if this is also required.
https://github.com/pytorch/pytorch/blob/ac8c6d09d1ce0a239f97473ebc01f1fe0ec87dcd/torch/csrc/tensor/python_tensor.cpp#L146
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78887
Approved by: https://github.com/ezyang
