Review naming conventions in io.image *_cpu methods

[datumbox]

🐛 Bug

The io.image namespace in C++ uses the old *_cpu postfix on the methods and files. This naming convention has been removed from ops in favour of *_kernel. Given that it's very likely soon we will have CUDA implementations for io.images, it's worth fixing the naming convention.

There are a few options:

1. Rename all files in io.image and methods from _cpu to _impl. This is one approach used in PyTorch in some methods.
2. Do the above but also rename all operators from _kernel to _impl.

cc @fmassa @vfdev-5