Fix issue where distribution_strategy="off" did not work.

Also improve error message when distribution=off is specified without properly quoting "off"

PiperOrigin-RevId: 359395329

official/common/distribute_utils.py

@@ -127,6 +127,15 @@ def get_distribution_strategy(distribution_strategy="mirrored",
   if num_gpus < 0:
     raise ValueError("`num_gpus` can not be negative.")
 
+  if not isinstance(distribution_strategy, str):
+    msg = ("distribution_strategy must be a string but got: %s." %
+           (distribution_strategy,))
+    if distribution_strategy == False:  # pylint: disable=singleton-comparison,g-explicit-bool-comparison
+      msg += (" If you meant to pass the string 'off', make sure you add "
+              "quotes around 'off' so that yaml interprets it as a string "
+              "instead of a bool.")
+    raise ValueError(msg)
+
   distribution_strategy = distribution_strategy.lower()
   if distribution_strategy == "off":
     if num_gpus > 1:

official/common/distribute_utils_test.py

@@ -41,6 +41,19 @@ class GetDistributionStrategyTest(tf.test.TestCase):
     for device in ds.extended.worker_devices:
       self.assertIn('GPU', device)
 
+  def test_no_strategy(self):
+    ds = distribute_utils.get_distribution_strategy('off')
+    self.assertIsNone(ds)
+
+  def test_invalid_strategy(self):
+    with self.assertRaisesRegexp(
+        ValueError,
+        'distribution_strategy must be a string but got: False. If'):
+      distribute_utils.get_distribution_strategy(False)
+    with self.assertRaisesRegexp(
+        ValueError, 'distribution_strategy must be a string but got: 1'):
+      distribute_utils.get_distribution_strategy(1)
+
 
 if __name__ == '__main__':
   tf.test.main()