[CPU]Avoid repeated random sample compile (#28260)

Signed-off-by: Zhang Xiangze <Xiangze.Zhang@arm.com>

vllm/v1/sample/ops/topk_topp_sampler.py

@@ -127,15 +127,6 @@ class TopKTopPSampler(nn.Module):
         elif self.logprobs_mode == "processed_logprobs":
             logits_to_return = logits.log_softmax(dim=-1, dtype=torch.float32)
 
-        # Note: this is a workaround for
-        # https://github.com/pytorch/pytorch/pull/151218
-        @torch.compile(dynamic=True)
-        def compiled_random_sample(logits: torch.Tensor) -> torch.Tensor:
-            probs = logits.softmax(dim=-1, dtype=torch.float32)
-            q = torch.empty_like(probs)
-            q.exponential_()
-            return probs.div(q).argmax(dim=-1).view(-1)
-
         if len(generators) != logits.shape[0]:
             return compiled_random_sample(logits), logits_to_return
         else:
@@ -148,6 +139,16 @@ class TopKTopPSampler(nn.Module):
             return probs.div_(q).argmax(dim=-1).view(-1), logits_to_return
 
 
+# Note: this is a workaround for
+# https://github.com/pytorch/pytorch/pull/151218
+@torch.compile(dynamic=True)
+def compiled_random_sample(logits: torch.Tensor) -> torch.Tensor:
+    probs = logits.softmax(dim=-1, dtype=torch.float32)
+    q = torch.empty_like(probs)
+    q.exponential_()
+    return probs.div(q).argmax(dim=-1).view(-1)
+
+
 def apply_top_k_top_p(
     logits: torch.Tensor,
     k: torch.Tensor | None,