ggml : general conv_2d CPU implementation (#352)
* Conv2d s0 == s1 == 1, d0 == d1 == 1, variable padding
* Mark unused varibles
* Support variable strides
* Handle all non-kernel-width convolutions with same general conv2d
* General 2d Conv
* Remove old function
* Group functions
* Rearrange
* General Conv2d implementation
* Clean up whitespace
