卷积操作的三种 padding：valid (no padding), same (or half), full
Valid or no padding:valid顾名思义filter只在valid（有效）区域卷积。如果stride=1，output_size = input_size -( kernel_size -1)Same or half padding:这种padding是让输出尺寸等于输入尺寸的padding。当stride = 1，要补（kernel_size - 1）行（列）零Full padding:Full意味着kernel会一个个遍历所有输入图像像素。当stride = ..
