[Fix] Support cls pooling in ModernBertPooler (#20067)

Signed-off-by: shengzhe.li <shengzhe.li@sbintuitions.co.jp>

Author: lsz05

vllm/model_executor/models/modernbert.py

@@ -258,14 +258,21 @@ def __init__(self, config: ModernBertConfig):
         super().__init__()
         self.dense = nn.Linear(config.hidden_size, config.hidden_size,
                                config.classifier_bias)
+        self.pooling_type = config.classifier_pooling
         self.act = nn.GELU()
         self.norm = nn.LayerNorm(config.hidden_size,
                                  eps=config.norm_eps,
                                  bias=config.norm_bias)
 
     def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:
         pooled_output = hidden_states
-        pooled_output = pooled_output.mean(dim=0, keepdim=False)
+        if self.pooling_type == "mean":
+            pooled_output = pooled_output.mean(dim=0, keepdim=False)
+        elif self.pooling_type == "cls":
+            pooled_output = pooled_output[0, :]
+        else:
+            raise ValueError("Pooling type should be either `cls` or `mean`, "
+                             f"but got {self.pooling_type}")
         pooled_output = self.norm(self.act(self.dense(pooled_output)))
         return pooled_output