Performance issues in retrain.py(P2)

[DLPerf]

Hello,I found a performance issue in the definition of build_eval_session ,
retrain.py,
eval_sess = tf.Session(graph=eval_graph) was repeatedly called and was not closed.
I think it will increase the efficiency and avoid out of memory if you close this session after using it.

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.

[yarodevuci]

@DLPerf can you make PR?