Adjust ncf_keras_benchmark to force TF 2.0 when running internally.

PiperOrigin-RevId: 267946336

official/recommendation/ncf_keras_benchmark.py

@@ -47,6 +47,7 @@ class NCFKerasBenchmarkBase(tf.test.Benchmark):
 
   def _setup(self):
     """Sets up and resets flags before each test."""
+    assert tf.version.VERSION.startswith('2.')
     tf.compat.v1.logging.set_verbosity(tf.compat.v1.logging.DEBUG)
     if NCFKerasBenchmarkBase.local_flags is None:
       ncf_common.define_ncf_flags()