[tf contrib seq2seq] Provide informative error messages in AttentionWrapper.call.

Related issues: #11077, #11540.

PiperOrigin-RevId: 162362081

tensorflow/contrib/seq2seq/python/ops/attention_wrapper.py

@@ -1137,7 +1137,14 @@ class AttentionWrapper(rnn_cell_impl.RNNCell):
       - `attention_or_cell_output` depending on `output_attention`.
       - `next_state` is an instance of `AttentionWrapperState`
          containing the state calculated at this time step.
+
+    Raises:
+      TypeError: If `state` is not an instance of `AttentionWrapperState`.
     """
+    if not isinstance(state, AttentionWrapperState):
+      raise TypeError("Expected state to be instance of AttentionWrapperState. "
+                      "Received type %s instead."  % type(state))
+
     # Step 1: Calculate the true inputs to the cell based on the
     # previous attention value.
     cell_inputs = self._cell_input_fn(inputs, state.attention)