test=develop, fix con2d with no bias (#17516)

* test=develop, fix con2d with no bias

* test=develop, fix conv_2d with no bias will have null shape

python/paddle/fluid/dygraph/nn.py

@@ -229,15 +229,17 @@ class Conv2D(layers.Layer):
                 'use_mkldnn': False,
             })
 
+        if self._bias_param is not None:
             pre_act = self._helper.create_variable_for_type_inference(
                 dtype=self._dtype)
-
             self._helper.append_op(
                 type='elementwise_add',
                 inputs={'X': [pre_bias],
                         'Y': [self._bias_param]},
                 outputs={'Out': [pre_act]},
                 attrs={'axis': 1})
+        else:
+            pre_act = pre_bias
 
         # Currently, we don't support inplace in dygraph mode
         return self._helper.append_activation(pre_act, act=self._act)