Performance issues in model.py(P2)

[DLPerf]

Hello,I found a performance issue in the definition of eval ,
model.py,
self.sess = tf.Session() was repeatedly called in for idx in range(data_num): and was not closed.
I think it will increase the efficiency and avoid out of memory if you close this session after using it.

The same issues also exist in self.sess = tf.Session()

Here are two files to support this issue,support1 and support2

Looking forward to your reply. Btw, I am very glad to create a PR to fix it if you are too busy.