[Bugfix](xpu): prevent “selected index k out of range” in TP decode path (#37259)

Signed-off-by: zhenzhao <zhenzhao@habana.ai>

vllm/_xpu_ops.py

@@ -426,7 +426,8 @@ class xpu_ops:
         mask = positions <= index_end_pos
         # mask: [B * N, L]
         logits = logits.masked_fill(~mask, float("-inf"))
-        topk_indices = logits.topk(topk_tokens, dim=-1)[1].to(torch.int32)  # [B * N, K]
+        real_topk = min(topk_tokens, logits.shape[-1])
+        topk_indices = logits.topk(real_topk, dim=-1)[1].to(torch.int32)  # [B * N, K]
         # ensure we don't set indices for the top k
         # that is out of range(masked already)
         # this will happen if context length is shorter than K