fix bias

test=develop

python/paddle/fluid/imperative/nn.py

@@ -261,9 +261,7 @@ class FC(layers.Layer):
             attrs={"use_mkldnn": False})
 
         bias_attr = self._helper.bias_attr
-        if not bias_attr:
-            return
-
+        if bias_attr:
             # add bias
             size = list(out.shape[1:])
             if not self._built:
@@ -277,5 +275,6 @@ class FC(layers.Layer):
                         'Y': [self._b]},
                 outputs={'Out': [bias_out]},
                 attrs={'axis': 1})
+            out = bias_out
         # add activation
-        return self._helper.append_activation(bias_out)
+        return self._helper.append_activation(out)