polish the cast op doc (#16078)

* polish the cast op doc

test=develop

* follow comments

test=develop

* fix api.spec

test=develop

paddle/fluid/API.spec

@@ -239,7 +239,7 @@ paddle.fluid.layers.load (ArgSpec(args=['out', 'file_path', 'load_as_fp16'], var
 paddle.fluid.layers.create_tensor (ArgSpec(args=['dtype', 'name', 'persistable'], varargs=None, keywords=None, defaults=(None, False)), ('document', 'c0c3d0194f83fff8ea99ce0820657dae'))
 paddle.fluid.layers.create_parameter (ArgSpec(args=['shape', 'dtype', 'name', 'attr', 'is_bias', 'default_initializer'], varargs=None, keywords=None, defaults=(None, None, False, None)), ('document', 'd62b866c899bc1fedb5385f95b88e1f8'))
 paddle.fluid.layers.create_global_var (ArgSpec(args=['shape', 'value', 'dtype', 'persistable', 'force_cpu', 'name'], varargs=None, keywords=None, defaults=(False, False, None)), ('document', 'ab914fac893607e29ac6e52bbdbea1a4'))
-paddle.fluid.layers.cast (ArgSpec(args=['x', 'dtype'], varargs=None, keywords=None, defaults=None), ('document', '60cb8f843d625abf33f8bf12455b8f99'))
+paddle.fluid.layers.cast (ArgSpec(args=['x', 'dtype'], varargs=None, keywords=None, defaults=None), ('document', '992eb42590fc1c380841a6db72ce78b3'))
 paddle.fluid.layers.tensor_array_to_tensor (ArgSpec(args=['input', 'axis', 'name'], varargs=None, keywords=None, defaults=(1, None)), ('document', 'b12717d3d4567e6119589f7f655b0cbb'))
 paddle.fluid.layers.concat (ArgSpec(args=['input', 'axis', 'name'], varargs=None, keywords=None, defaults=(0, None)), ('document', 'b19b79be4f05e85d1d6cec642c9fb535'))
 paddle.fluid.layers.sums (ArgSpec(args=['input', 'out'], varargs=None, keywords=None, defaults=(None,)), ('document', '42912092418620b4be07f36af31e7816'))

paddle/fluid/operators/cast_op.cc

@@ -13,6 +13,7 @@ See the License for the specific language governing permissions and
 limitations under the License. */
 
 #include "paddle/fluid/operators/cast_op.h"
+#include <memory>
 #include "paddle/fluid/framework/op_registry.h"
 #include "paddle/fluid/platform/float16.h"
 
@@ -30,7 +31,8 @@ class CastOpProtoMaker : public framework::OpProtoAndCheckerMaker {
 Cast Operator.
 
 This Operator casts the input tensor to another data type and
-returns tha Output Tensor.
+returns the Output Tensor. It's meaningless if the output dtype equals
+the input dtype, but it's fine if you do so.
 
 )DOC");
   }

python/paddle/fluid/layers/tensor.py

@@ -142,7 +142,8 @@ def create_global_var(shape,
 def cast(x, dtype):
     """
     This layer takes in the Variable :attr:`x` with :attr:`x.dtype` and casts
-    it to the output with :attr:`dtype`.
+    it to the output with :attr:`dtype`. It's meaningless if the output
+    dtype equals the input dtype, but it's fine if you do so.
 
     Args:
         x (Variable): The input Variable for casting.