the size of input tensor have to > 26*26 !!!

[victor000000]

for the stride 2 conv in class SPP_NET,
we have to ensure that the size of tensor > 26 * 26

otherwise, the kernal(4 * 4) of self.conv5 will larger than the conv5 input
i.e. the conv5 input will < 4 * 4 if the input x in forward < 26 * 26