[cherry-pick] Fix fp16 in input.dtype check in layers.fc (#20467) test=develop, (#20500)

test=release/1.6

* Add fp16 in input.dtype check test=develop

* Add warning of fp16 in CPU test=develop

* add unittest code for fp16 test=develop

* fix float16 list error test=develop

python/paddle/fluid/layers/nn.py

@@ -355,9 +355,12 @@ def fc(input,
                 "The type of 'input' in fc must be Variable, but received %s" %
                 (type(input)))
     dtype = helper.input_dtype()
-    if convert_dtype(dtype) not in ['float32', 'float64']:
+    if convert_dtype(dtype) in ['float16']:
+        warnings.warn(
+            "The data type of 'input' in fc only support float16 in GPU now.")
+    if convert_dtype(dtype) not in ['float16', 'float32', 'float64']:
         raise TypeError(
-            "The data type of 'input' in fc must be float32 or float64, but received %s."
+            "The data type of 'input' in fc must be float16, float32 or float64, but received %s."
             % (convert_dtype(dtype)))
 
     mul_results = []

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

@@ -148,6 +148,10 @@ class TestFCOpError(OpTest):
 
             self.assertRaises(TypeError, test_type)
 
+            # The input dtype of fc can be float16 in GPU, test for warning
+            x3 = fluid.layers.data(name='x3', shape=[4], dtype='float16')
+            fluid.layers.fc(input=x3, size=1)
+
 
 if __name__ == "__main__":
     unittest.main()