Performance issue in the definition of create_tf_model, tests/perf/measure_runtimes.py(P1)

[DLPerf]

Hello, I found a performance issue in the definition of create_tf_model, tests/perf/measure_runtimes.py, b = tf.Variable(tf.zeros([units])) will be created repeatedly during program execution, resulting in reduced efficiency. I think it should be created before the loop.

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