[bugfix] fixed parameter “n” when set parameter “bestof” > 1 (#10854)

Signed-off-by: jianzheng <57654625+o2363286@users.noreply.github.com>

vllm/sampling_params.py

@@ -293,6 +293,7 @@ class SamplingParams(
                 raise ValueError(
                     f"best_of must be greater than or equal to n, "
                     f"got n={self.n} and best_of={self.best_of}.")
+            if not self._real_n:
                 self._real_n = self.n
                 self.n = self.best_of