Fix AttributeError in resnet.py

There is no function tf.softmax() in Tensorflow 1.x.

When running the old code, Python interpreter complains:

File "resnet.py", line 152, in res_net_model
prediction, loss = res_net(x, y)
File "resnet.py", line 148, in res_net
return tf.softmax(logits), loss
AttributeError: 'module' object has no attribute 'softmax'