Fix SAC + LSTM Barracuda inference (#2698)

ml-agents/mlagents/trainers/sac/models.py

         )
         # We assume m_size is divisible by 4
         # Create the non-Policy inputs
+        # Use a default placeholder here so nothing has to be provided during
+        # Barracuda inference. Note that the default value is just the tiled input
+        # for the policy, which is thrown away.
-        self.other_memory_in = tf.placeholder(
+        self.other_memory_in = tf.placeholder_with_default(
+            input=tf.tile(self.inference_memory_in, [1, 3]),
-            dtype=tf.float32,
             name="other_recurrent_in",
         )