[BugFix] fix unexpected exception when agg_type is GCN and bias disabled (#5127)

* [BugFix] fix unexpected exception when agg_type is GCN and bias is disabled

* Update sageconv.py
Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>

python/dgl/nn/pytorch/conv/sageconv.py

@@ -129,6 +129,8 @@ class SAGEConv(nn.Module):
             self.fc_self = nn.Linear(self._in_dst_feats, out_feats, bias=bias)
         elif bias:
             self.bias = nn.parameter.Parameter(torch.zeros(self._out_feats))
+        else:
+            self.register_buffer("bias", None)
 
         self.reset_parameters()