[V1] Fix: make sure k_index is int64 for apply_top_k_only (#15907)

Signed-off-by: Brayden Zhong <b8zhong@uwaterloo.ca>

Author: b8zhong

vllm/v1/sample/ops/topk_topp_sampler.py

@@ -200,7 +200,7 @@ def apply_top_k_only(
     # topk.values tensor has shape [batch_size, max_top_k].
     # Convert top k to 0-based index in range [0, max_top_k).
     k_index = k.sub_(1).unsqueeze(1)
-    top_k_mask = logits.topk(max_top_k, dim=1).values.gather(1, k_index)
+    top_k_mask = logits.topk(max_top_k, dim=1).values.gather(1, k_index.long())
     # Handle non-topk rows.
     top_k_mask.masked_fill_(no_top_k_mask.unsqueeze(1), -float("inf"))
     logits.masked_fill_(logits < top_k_mask, -float("inf"))