[Core] Factor out input preprocessing to a separate class (vllm-project#7329)

Signed-off-by: Amit Garg <mitgarg17495@gmail.com>

Author: DarkLight1337

vllm/engine/async_llm_engine.py

@@ -14,9 +14,7 @@
 from vllm.engine.metrics_types import StatLoggerBase
 from vllm.executor.executor_base import ExecutorAsyncBase
 from vllm.executor.ray_utils import initialize_ray_cluster
-from vllm.inputs import (EncoderDecoderLLMInputs, LLMInputs, PromptInputs,
-                         SingletonPromptInputs)
-from vllm.inputs.parse import is_explicit_encoder_decoder_prompt
+from vllm.inputs import PromptInputs
 from vllm.logger import init_logger
 from vllm.lora.request import LoRARequest
 from vllm.model_executor.layers.sampler import SamplerOutput