Add warnings/errors on CPU kernels when called with GPU arrays

https://github.com/FluxML/model-zoo/issues/400 is but the most recent example of where this would help. Now that we have package extensions, this should be pretty trivial. In total there are probably less than a dozen conv + pooling functions to cover, I think.

Start by reading the linked FluxML/model-zoo issue and locating the CPU convolution and pooling entry points in NNlib.jl. Check how package extensions distinguish CPU and GPU arrays, then cover the mentioned functions with warnings or errors for GPU-array calls; done means the behavior is applied consistently across the listed kernels and verified by tests.

Written by the indexing model from the issue text.