Improve fast_topk function with type hints and documentation (#22530)

Signed-off-by: zitian.zhao <zitian.zhao@tencentmusic.com>

vllm/model_executor/models/utils.py

@@ -736,7 +736,23 @@ def cast_overflow_tensors(
     return tensors
 
 
-def fast_topk(values, topk, dim):
+def fast_topk(values: torch.Tensor, topk: int,
+              dim: int) -> tuple[torch.Tensor, torch.Tensor]:
+    """
+    Optimized topk implementation that uses torch.max for k=1 case.
+    
+    This function provides better performance for the common case of k=1
+    by using torch.max instead of the more general torch.topk.
+    
+    Args:
+        values: Input tensor to find top-k values from
+        topk: Number of top values to return (k). Must be > 0.
+        dim: Dimension along which to compute topk
+        
+    Returns:
+        Tuple of (values, indices) where values are the top-k values
+        and indices are their corresponding indices in the input tensor
+    """
     if topk == 1:
         # Use max along the specified dimension to get both value and index
         return torch.max(values, dim=dim, keepdim=True)