Performance issue in Appendix/pix2pix_v2_keras.py

[DLPerf]

Hello! Our static bug checker has found a performance issue in Appendix/pix2pix_v2_keras.py: Python type argument epoch is passed to tf.function decorated function train_step 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.

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! @solaris33