Should not modify index in im2col kernel loop

I assume this is a mistake, although I don't think it actually breaks anything in practice with the launch configuration used.
Minor comment: maybe suppressing the above 4 lines to 2:

Dtype* data_col_ptr = data_col + (channel_out * height_col + h_out) * width_col + w_out;
const Dtype* data_im_ptr = data_im + (channel_in * height + h_in) * width + w_in;