Auto-select conv2d implementation based on kernel/input size

[rmatif]

Since #14316, #14388, and #14403 will add a new conv2d direct op, I'm considering if we can add a mechanism that automatically chooses the best method during graph construction, it could decide between the direct ggml_conv_2d op and the indirect im2col+gemm method based on kernel or input size (determined heuristically with hardcoded thresholds). This would build the most performant path. Or maybe a wrapper by implementing a new op like GGML_OP_CONV_2D_AUTO that will call either direct or direct op based on input/kernel, still not sure le on how to do it.

@slaren I'd like to know your thoughts on this. It's a standard practice in other frameworks, but I'm not sure if it's desirable here