MPS - Support for Convolutional Neural Network kernels

[4SAnalyticsnModelling]

Cross posting from Flux.jl FluxML/Flux.jl#2278

[stemann]

... someone needs to add support for https://developer.apple.com/documentation/metalperformanceshaders/convolutional_neural_network_kernels and plumb it through to NNlib.jl.

Additional starting points (to start out with something basic - first releases with MPS):

• Header files for the first macOS SDK to include the MPS Framework: MacOSX10.13.sdk: /System/Library/Frameworks/MetalPerformanceShaders.framework/Frameworks/MPSNeuralNetwork.framework/Headers
• PyTorch application of MPS (first release with MPS): PyTorch v1.8.0: aten/src/ATen/native/metal/mpscnn