[Bugfix] remove post_layernorm in siglip (#8106)

Author: wnma3mz

vllm/model_executor/models/siglip.py

@@ -443,14 +443,27 @@ def __init__(
         self.config = config
         embed_dim = config.hidden_size
 
+        if (num_hidden_layers_override is None
+                or num_hidden_layers_override == config.num_hidden_layers):
+            self.need_post_layernorm = True
+        elif num_hidden_layers_override > config.num_hidden_layers:
+            raise ValueError(
+                "num_hidden_layers_override cannot be greater than "
+                "num_hidden_layers")
+        else:
+            self.need_post_layernorm = False
+
         self.embeddings = SiglipVisionEmbeddings(config)
         self.encoder = SiglipEncoder(
             config,
             quant_config=quant_config,
             num_hidden_layers_override=num_hidden_layers_override,
         )
-        self.post_layernorm = nn.LayerNorm(embed_dim,
-                                           eps=config.layer_norm_eps)
+        if self.need_post_layernorm:
+            self.post_layernorm = nn.LayerNorm(embed_dim,
+                                               eps=config.layer_norm_eps)
+        else:
+            self.post_layernorm = nn.Identity()
         self.use_head = (True if not hasattr(config, "vision_use_head") else
                          config.vision_use_head)
         if self.use_head:
@@ -470,7 +483,6 @@ def forward(
         encoder_outputs = self.encoder(inputs_embeds=hidden_states)
 
         last_hidden_state = self.post_layernorm(encoder_outputs)
-
         # TODO: add this back when pooled_output is used in inference
         # if self.use_head:
         # pooled_output = self.head(last_hidden_state)
@@ -499,6 +511,10 @@ def __init__(
             num_hidden_layers_override=num_hidden_layers_override,
         )
 
+    @property
+    def need_post_layernorm(self):
+        return self.vision_model.need_post_layernorm
+
     def get_input_embeddings(self) -> nn.Module:
         return self.vision_model.embeddings.patch_embedding
 
@@ -517,6 +533,11 @@ def load_weights(self, weights: Iterable[Tuple[str, torch.Tensor]]):
         layer_count = len(self.vision_model.encoder.layers)
 
         for name, loaded_weight in weights:
+            # post_layernorm is optional in SiglipVisionModel
+            if ("vision_model.post_layernorm" in name
+                    and not self.need_post_layernorm):
+                continue
+
             # omit layers when num_hidden_layers_override is set
             if "vision_model.encoder.layers." in name:
                 layer_idx = int(name.split(".")[3])