fix local variable 'response' referenced before assignment in async_engine.generate

[irexyc]

Motivation

When input is long or the chat template doesn't match, the first output token id with turbomind backend may be eos_id or in stop_words. In this case, the async_engine.generate will meet the following error.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/chenxin/ws3/vl/lmdeploy/serve/async_engine.py", line 325, in __call__
    return self.batch_infer(prompts,
  File "/home/chenxin/ws3/vl/lmdeploy/serve/async_engine.py", line 441, in batch_infer
    _get_event_loop().run_until_complete(gather())
  File "/home/chenxin/miniconda3/envs/38/lib/python3.8/asyncio/base_events.py", line 616, in run_until_complete
    return future.result()
  File "/home/chenxin/ws3/vl/lmdeploy/serve/async_engine.py", line 436, in gather
    await asyncio.gather(*[
  File "/home/chenxin/ws3/vl/lmdeploy/serve/async_engine.py", line 423, in _inner_call
    async for out in generator:
  File "/home/chenxin/ws3/vl/lmdeploy/serve/async_engine.py", line 648, in generate
    if not response.endswith('�'):
UnboundLocalError: local variable 'response' referenced before assignment

reproduce

from lmdeploy import pipeline
pipe = pipeline('/mnt/140/Qwen/Qwen1.5-7B-Chat')
pipe('hello ' * 7500)

[zhulinJulia24]

Special tokens have been added in the vocabulary, make sure the associated word embeddings are fine-tuned or trained.
[WARNING] gemm_config.in is not found; using default GEMM algo
Response(text='', generate_token_len=1, input_token_len=7520, session_id=0, finish_reason='stop', token_ids=[], logprobs=None)

fixed

[zhulinJulia24]

lgtm