grpc_server: fix tokenizer group usage

[dtrifiro]

fixes the following crash on startup

ERROR:    Traceback (most recent call last):
  File "/opt/vllm/lib/python3.11/site-packages/starlette/routing.py", line 732, in lifespan
    async with self.lifespan_context(app) as maybe_state:
  File "/opt/vllm/lib/python3.11/contextlib.py", line 210, in __aenter__
    return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/vllm/entrypoints/openai/api_server.py", line 50, in lifespan
    grpc_server = await start_grpc_server(async_llm_engine, args)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/vllm/entrypoints/grpc/grpc_server.py", line 611, in start_grpc_server
    await service._post_init()
  File "/workspace/vllm/entrypoints/grpc/grpc_server.py", line 106, in _post_init
    self.tokenizer_group = await self.engine.get_tokenizer_group()
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'AsyncLLMEngine' object has no attribute 'get_tokenizer_group'

ERROR:    Application startup failed. Exiting.

[z103cb]

/lgtm

[z103cb]

/approve

[openshift-ci]

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This pull-request has been approved by: dtrifiro, z103cb

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