Performance issue in the definition of _sliced_wasserstein, tensorflow_gan/python/eval/sliced_wasserstein.py

[DLPerf]

Hello, I found a performance issue in the definition of _sliced_wasserstein, tensorflow_gan/python/eval/sliced_wasserstein.py, tf.shape(input=a)[1] will calculated repeatedly during the program execution, resulting in reduced efficiency. I think it should be created before the loop in the _sliced_wasserstein.

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