Performance issues about tf.function

[DLPerf]

Hello! Our static bug checker has found a performance issue in imitation_learning/train_bc.py: Python type argument epoch is passed to tf.function decorated function train instead of tensors. As there is a for loop, every time the value changes, a new graph will be generated, and that can trigger tf.function retracing warning.

Similar issue in cmvae/train_cmvae.py.

Here is the tensorflow document to support it.

Looking forward to your reply.

[DLPerf]

We are investigating this kind of issues, and your answer will be of great help to our work. Can you take a look? Thank you in advance! @sverrejoh @radical