Fix _print_stream crash when the stream is empty - #794
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The streaming loop referenced the loop variable `state` after the loop, so an empty iterator (e.g. max_new_tokens=0 or a cache that is already full) raised UnboundLocalError instead of a useful error. Track the last state and raise a clear ValueError when nothing was generated.
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ChatSampler 开 print_stream 时，如果模型一个 token 都没生成（max_new_tokens=0，或者 cache_length 太小一上来就满了），
_print_stream会在循环结束后去用没绑定的循环变量state，直接抛 UnboundLocalError，用户根本看不出是哪的问题。改成在循环里记下最后一个 state，流是空的话抛一条明确的 ValueError，提示大概率是 max_new_tokens=0 或 cache 满了（建议加大 cache_length）。非空流的行为完全不变。
顺带把之前压着这个问题的
# pylint: disable=undefined-variable,undefined-loop-variable去掉了，因为现在这个 bug 真正被处理了。