Fix rnn grad bug in cpu when dropout is zero (PaddlePaddle#37080)

* fix rnn grad bug when num_layers is set 2 and dropout_prob is set 0

* add more test for rnn

paddle/fluid/operators/rnn_op.h

@@ -965,6 +965,9 @@ class RNNCPUKernel : public framework::OpKernel<T> {
     }
     dropout_mask->mutable_data<uint8_t>(output->dims(), ctx.GetPlace());
 
+    auto& dev_ctx = ctx.template device_context<platform::CPUDeviceContext>();
+    math::SetConstant<platform::CPUDeviceContext, uint8_t> ones;
+    ones(dev_ctx, dropout_mask, static_cast<uint8_t>(1));
     // init the output and allocate the memory
     output->mutable_data<T>(ctx.GetPlace());
     int gate_num = 4;

python/paddle/fluid/tests/unittests/test_rnn_op.py

@@ -168,5 +168,35 @@ def set_attrs(self):
         self.is_bidirec = True
 
 
+class TestRNNOp5(TestRNNOp):
+    def set_attrs(self):
+        self.num_layers = 2
+
+
+class TestRNNOp6(TestRNNOp):
+    def set_attrs(self):
+        self.num_layers = 2
+        self.is_bidirec = True
+
+
+class TestRNNOp7(TestRNNOp):
+    def set_attrs(self):
+        self.num_layers = 2
+        self.is_bidirec = True
+        self.is_test = True
+
+
+class TestRNNOp8(TestRNNOp):
+    def set_attrs(self):
+        self.num_layers = 2
+        self.is_bidirec = True
+        self.sequence_length = None
+
+
+class TestRNNOp9(TestRNNOp):
+    def set_attrs(self):
+        self.num_layers = 3
+
+
 if __name__ == '__main__':
     unittest.main()