fix tensorflow fast decode bugs

tensor2tensor/models/transformer.py

@@ -34,6 +34,7 @@ from tensor2tensor.models import common_layers
 from tensor2tensor.utils import registry
 from tensor2tensor.utils import t2t_model
 from tensor2tensor.utils import beam_search
+from tensorflow.python.util import nest
 
 import tensorflow as tf
 
@@ -409,7 +410,7 @@ def fast_decode(encoder_output,
             tf.TensorShape([None]),
             tf.TensorShape([None, None]),
             tf.TensorShape([None, None]),
-            nest.map_structure(beam_search_slow.get_state_shape_invariants, cache),
+            nest.map_structure(beam_search.get_state_shape_invariants, cache),
             tf.TensorShape([None]),
         ])
     scores = log_prob