Allow users passing master argument to avoid instantiating a TPUClusterResolver (#3845)
Allow users passing master argument to avoid instantiating a TPUClusterResolver (#3845)

* Fix MNIST to allow master=local to work again.

* Update mnist_tpu.py

cshallue/models,tombstone/models,cshallue/models,derekjchow/models,cshallue/models,derekjchow/models,tombstone/models,derekjchow/models,derekjchow/models,tombstone/models,alexgorban/models,alexgorban/models,alexgorban/models,derekjchow/models,tombstone/models,tombstone/models,alexgorban/models,tombstone/models,alexgorban/models,cshallue/models,cshallue/models