!15288 Fix check string attr of DynamicRnn.

From: @liu_xiao_93
Reviewed-by: @liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui

mindspore/ops/operations/nn_ops.py

@@ -7261,8 +7261,11 @@ class DynamicRNN(PrimitiveWithInfer):
         self.use_peephole = validator.check_value_type("use_peephole", use_peephole, [bool], self.name)
         self.time_major = validator.check_value_type("time_major", time_major, [bool], self.name)
         self.is_training = validator.check_value_type("is_training", is_training, [bool], self.name)
+        validator.check_value_type("cell_type", cell_type, [str], self.name)
         self.cell_type = validator.check_string(cell_type, ['LSTM'], "cell_type", self.name)
+        validator.check_value_type("direction", direction, [str], self.name)
         self.direction = validator.check_string(direction, ['UNIDIRECTIONAL'], "direction", self.name)
+        validator.check_value_type("activation", activation, [str], self.name)
         self.activation = validator.check_string(activation, ['tanh'], "activation", self.name)
 
     def infer_shape(self, x_shape, w_shape, b_shape, seq_shape, h_shape, c_shape):