[V1][Bugfix] Always set enable_chunked_prefill = True for V1

[WoosukKwon]

This PR fixes a bug in initializing the OpenAI server with V1: Currently, the server fails for some models with the error message: ValueError: max_num_batched_tokens (2048) is smaller than max_model_len (8192). This is because enable_chunked_prefill is not properly set.

[github-actions]

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