### What this PR does / why we need it?
Align with upstream vLLM. This PR will help downstream vLLM-Omni reduce
the cost for maintaining the _prepare_inputs. Besides, it helps
vLLM-Ascend code more readable. In the future, we can follow closer to
vLLM. The `preprocess` logic is same as GPUModelRunner. We don't need to
maintain it anymore.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
CI.
- vLLM version: v0.14.0
- vLLM main:
d68209402d
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Signed-off-by: gcanlin <canlinguosdu@gmail.com>