[bugfix] Fix PPMissingLayer AttributeError when Using PP (#19804)

python/sglang/srt/layers/utils/common.py

@@ -59,12 +59,28 @@ class PPMissingLayer(torch.nn.Identity):
     # https://github.com/vllm-project/vllm/blob/18ed3132d2bfe1df9a74729457b69243955221e8/vllm/model_executor/models/utils.py#L468C1-L486C1
     """
     A placeholder layer for missing layers in a pipeline parallel model.
+
+    Attribute access on this placeholder returns 'self' so that common
+    patterns like ``isinstance(layer.mlp, SomeMoE)`` safely evaluate to
+    ``False`` instead of raising ``AttributeError``.
     """
 
     def __init__(self, *args, **kwargs):
         super().__init__()
         self.return_tuple = kwargs.get("return_tuple", False)
 
+    def __getattr__(self, name: str):
+        try:
+            return super().__getattr__(name)
+        except (AttributeError, KeyError):
+            return self
+
+    def __setattr__(self, name: str, value):
+        if name == "return_tuple" or name.startswith("_"):
+            super().__setattr__(name, value)
+        else:
+            pass
+
     def forward(self, *args, **kwargs):
         """
         Return the first arg from args or the first value from kwargs.