Fix sequence_length when None for RNN

hapi/text/text.py

@@ -818,7 +818,7 @@ class RNN(fluid.dygraph.Layer):
                     lambda x: fluid.layers.transpose(x, [1, 0] + list(
                         range(2, len(x.shape)))), inputs)
 
-            if sequence_length:
+            if sequence_length is not None:
                 mask = fluid.layers.sequence_mask(
                     sequence_length,
                     maxlen=time_steps,
@@ -829,7 +829,7 @@ class RNN(fluid.dygraph.Layer):
                 inputs = map_structure(
                     lambda x: fluid.layers.reverse(x, axis=[0]), inputs)
                 mask = fluid.layers.reverse(
-                    mask, axis=[0]) if sequence_length else None
+                    mask, axis=[0]) if sequence_length is not None else None
 
             states = initial_states
             outputs = []
@@ -837,7 +837,7 @@ class RNN(fluid.dygraph.Layer):
                 step_inputs = map_structure(lambda x: x[i], inputs)
                 step_outputs, new_states = self.cell(step_inputs, states,
                                                      **kwargs)
-                if sequence_length:
+                if sequence_length is not None:
                     new_states = map_structure(
                         partial(
                             _maybe_copy, step_mask=mask[i]),