Fix _forget_bias.stop_gradient workround in hapi.text.BasicLSTMCell. (#24655)

test=develop

python/paddle/incubate/hapi/text/text.py

@@ -254,7 +254,8 @@ class BasicLSTMCell(RNNCell):
         # TODO(guosheng): find better way to resolve constants in __init__
         self._forget_bias = layers.create_global_var(
             shape=[1], dtype=dtype, value=forget_bias, persistable=True)
-        self._forget_bias.stop_gradient = True
+        # TODO(guosheng): refine this if recurrent_op removes gradient require
+        self._forget_bias.stop_gradient = False
         self._dtype = dtype
         self._input_size = input_size