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

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

* Update mnist_tpu.py

official/mnist/mnist_tpu.py

@@ -46,6 +46,10 @@ tf.flags.DEFINE_string(
     "metadata.")
 
 # Model specific parameters
+tf.flags.DEFINE_string(
+    "master", default=None,
+    help="GRPC URL of the master (e.g. grpc://ip.address.of.tpu:8470). You "
+    "must specify either this flag or --tpu.")
 tf.flags.DEFINE_string("data_dir", "",
                        "Path to directory containing the MNIST dataset")
 tf.flags.DEFINE_string("model_dir", None, "Estimator model_dir")
@@ -132,11 +136,24 @@ def main(argv):
   del argv  # Unused.
   tf.logging.set_verbosity(tf.logging.INFO)
 
-  tpu_cluster_resolver = tf.contrib.cluster_resolver.TPUClusterResolver(
-      FLAGS.tpu, zone=FLAGS.tpu_zone, project=FLAGS.gcp_project)
+  if FLAGS.master is None and FLAGS.tpu is None:
+    raise RuntimeError('You must specify either --master or --tpu.')
+  if FLAGS.master is not None:
+    if FLAGS.tpu is not None:
+      tf.logging.warn('Both --master and --tpu are set. Ignoring '
+                      '--tpu and using --master.')
+    tpu_grpc_url = FLAGS.master
+  else:
+    tpu_cluster_resolver = (
+        tf.contrib.cluster_resolver.TPUClusterResolver(
+            FLAGS.tpu,
+            zone=FLAGS.tpu_zone,
+            project=FLAGS.gcp_project))
+    tpu_grpc_url = tpu_cluster_resolver.get_master()
 
   run_config = tf.contrib.tpu.RunConfig(
-      cluster=tpu_cluster_resolver,
+      master=tpu_grpc_url,
+      evaluation_master=tpu_grpc_url,
       model_dir=FLAGS.model_dir,
       session_config=tf.ConfigProto(
           allow_soft_placement=True, log_device_placement=True),