fix sign op input error check (#20473)

fix sign op input error check
test=release/1.6

python/paddle/fluid/layers/nn.py

@@ -16335,9 +16335,12 @@ def sign(x):
                 "The type of 'x' in sign_op must be Variable or numpy.ndarray, but received %s."
                 % (type(x)))
 
-    if convert_dtype(x.dtype) not in ['float32', 'float64']:
+    if convert_dtype(x.dtype) in ['float16']:
+        warnings.warn(
+            "The data type of 'x' in sign_op only support float16 in GPU now.")
+    if convert_dtype(x.dtype) not in ['float16', 'float32', 'float64']:
         raise TypeError(
-            "The data type of 'x' in sign_op must be float32 or float64, but received %s."
+            "The data type of 'x' in sign_op must be float16, float32 or float64, but received %s."
             % (convert_dtype(x.dtype)))
 
     out = helper.create_variable_for_type_inference(dtype=x.dtype)

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

@@ -42,10 +42,13 @@ class TestSignOpError(OpTest):
             # The input type of sign_op must be Variable or numpy.ndarray.
             input1 = 12
             self.assertRaises(TypeError, fluid.layers.sign, input1)
-            # The input dtype of sign_op must be float32, float64.
+            # The input dtype of sign_op must be float16, float32, float64.
             input2 = fluid.layers.data(
                 name='input2', shape=[12, 10], dtype="int32")
             self.assertRaises(TypeError, fluid.layers.sign, input2)
+            input3 = fluid.layers.data(
+                name='input3', shape=[4], dtype="float16")
+            fluid.layers.sign(input3)
 
 
 if __name__ == "__main__":