cherry-pick from #17935 (#18051)

test=release/1.5

* Fix the GetExpectedKernelType of add_position_encoding_op.

* Fix the doc of lstm_unit outputs in nn.py.

Author: guoshengCS

paddle/fluid/API.spec

@@ -104,7 +104,7 @@ paddle.fluid.layers.sequence_expand (ArgSpec(args=['x', 'y', 'ref_level', 'name'
 paddle.fluid.layers.sequence_expand_as (ArgSpec(args=['x', 'y', 'name'], varargs=None, keywords=None, defaults=(None,)), ('document', '0ecf8d24cf4fd5c035406ee46afccfa0'))
 paddle.fluid.layers.sequence_pad (ArgSpec(args=['x', 'pad_value', 'maxlen', 'name'], varargs=None, keywords=None, defaults=(None, None)), ('document', '6a1adf3067b20f6e4bcb354d71c19184'))
 paddle.fluid.layers.sequence_unpad (ArgSpec(args=['x', 'length', 'name'], varargs=None, keywords=None, defaults=(None,)), ('document', 'd12803c903c99aa36ec03aaac5f0cc5b'))
-paddle.fluid.layers.lstm_unit (ArgSpec(args=['x_t', 'hidden_t_prev', 'cell_t_prev', 'forget_bias', 'param_attr', 'bias_attr', 'name'], varargs=None, keywords=None, defaults=(0.0, None, None, None)), ('document', '409d1c2ca874a22511258175649d2b7f'))
+paddle.fluid.layers.lstm_unit (ArgSpec(args=['x_t', 'hidden_t_prev', 'cell_t_prev', 'forget_bias', 'param_attr', 'bias_attr', 'name'], varargs=None, keywords=None, defaults=(0.0, None, None, None)), ('document', 'fe126c58e4339410e875ab1eba246d21'))
 paddle.fluid.layers.reduce_sum (ArgSpec(args=['input', 'dim', 'keep_dim', 'name'], varargs=None, keywords=None, defaults=(None, False, None)), ('document', 'dd5f06fb7cf39ca06cbab4abd03e6893'))
 paddle.fluid.layers.reduce_mean (ArgSpec(args=['input', 'dim', 'keep_dim', 'name'], varargs=None, keywords=None, defaults=(None, False, None)), ('document', 'a3024789eba11a70c2ef27c358173400'))
 paddle.fluid.layers.reduce_max (ArgSpec(args=['input', 'dim', 'keep_dim', 'name'], varargs=None, keywords=None, defaults=(None, False, None)), ('document', '10023caec4d7f78c3b901f023a1feaa7'))

paddle/fluid/operators/add_position_encoding_op.cc

@@ -33,6 +33,13 @@ class AddPositionEncodingOp : public framework::OperatorWithKernel {
     ctx->SetOutputDim("Out", x_dims);
     ctx->ShareLoD("X", /*->*/ "Out");
   }
+
+ protected:
+  framework::OpKernelType GetExpectedKernelType(
+      const framework::ExecutionContext& ctx) const override {
+    return framework::OpKernelType(ctx.Input<framework::LoDTensor>("X")->type(),
+                                   platform::CPUPlace());
+  }
 };
 
 class AddPositionEncodingOpGrad : public framework::OperatorWithKernel {
@@ -45,6 +52,14 @@ class AddPositionEncodingOpGrad : public framework::OperatorWithKernel {
       ctx->SetOutputDim(framework::GradVarName("X"), out_dims);
     }
   }
+
+ protected:
+  framework::OpKernelType GetExpectedKernelType(
+      const framework::ExecutionContext& ctx) const override {
+    return framework::OpKernelType(
+        ctx.Input<framework::LoDTensor>(framework::GradVarName("Out"))->type(),
+        platform::CPUPlace());
+  }
 };
 
 class AddPositionEncodingOpMaker : public framework::OpProtoAndCheckerMaker {

python/paddle/fluid/layers/nn.py

@@ -4496,7 +4496,7 @@ def lstm_unit(x_t,
 
             i_t = \sigma(L_{i_t})
 
-    This layer has two outputs including :math:`h_t` and :math:`o_t`.
+    This layer has two outputs including :math:`h_t` and :math:`c_t`.
 
     Args:
         x_t (Variable): The input value of current step, a 2-D tensor with shape