ggml : Depthwise 2D convolution (ggml/1152)
* ggml-cpu : kernels for faster depthwise 2D convolution
* fix compile: remove static after moving to ops.cpp
* add dilation for depthwise_conv_2d
* review: rename to ggml_conv_2d_dw_direct, remove redundant struct keywords, pass by ref, whitespace
* review: rename depthwise_conv_2d -> conv_2d_dw everywhere
