### What this PR does / why we need it?
Refector prepare_inputs in model_runner_v1.py for more easy read.
### Does this PR introduce _any_ user-facing change?
NO
### How was this patch tested?
PASS CI
- vLLM version: v0.10.1.1
- vLLM main:
e599e2c65e
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Signed-off-by: ChenTaoyu-SJTU <ctynb@qq.com>