Revert "[Eager] Remove elementwise add in conv (PaddlePaddle#41515)"

This reverts commit 33abfbe.

python/paddle/nn/functional/conv.py

@@ -127,12 +127,8 @@ def _conv_nd(x,
             x, weight, stride, padding, padding_algorithm, groups, dilation,
             data_format, False, -1, False)
         if bias is not None:
-            channel_dim = channel_dim + len(
-                x.shape) if channel_dim < 0 else channel_dim
-            tmp_bias = _C_ops.final_state_reshape(
-                bias, bias.shape +
-                [1 for i in range(len(x.shape) - channel_dim - 1)])
-            return _C_ops.final_state_add(pre_bias, tmp_bias)
+            out = nn.elementwise_add(pre_bias, bias, axis=channel_dim)
+            return out
         else:
             return pre_bias
     if in_dynamic_mode():