add default value for label

tools/export_model.py

@@ -66,7 +66,7 @@ class IdentityHead(nn.Layer):
     def __init__(self):
         super(IdentityHead, self).__init__()
 
-    def forward(self, x, label):
+    def forward(self, x, label=None):
         return {"features": x, "logits": None}