message: TF2 tensor string difference fix
  Summary:
  Tensorflow2 describes a tensor with an unknown dimensions
  differently - support both descriptions
  Test Plan: test passes
  Reviewers: georgep
diff: @@ -392,7 +392,7 @@ class InfeedOutfeedTest(test_util.TensorFlowTestCase):
  def testUndefinedShape(self):
  dataset = tu.create_single_increasing_dataset(10, shape=[4, 4])
  dataset = dataset.batch(10, drop_remainder=False)
  - with self.assertRaisesRegexp(ValueError, 'Output shape \(\?,'):
  + with self.assertRaisesRegexp(ValueError, 'Output shape \((\?|None),'):
  infeed_queue = ipu.ipu_infeed_queue.IPUInfeedQueue(
  dataset, next_feed_id())
  