### What this PR does / why we need it?
Now there is no need to calculate `num_draft_tokens` when allocating
slots.
This PR follows the changes in vllm:
https://github.com/vllm-project/vllm/pull/20701
### Does this PR introduce _any_ user-facing change?
N/A
### How was this patch tested?
CI passed with existing test
- vLLM version: v0.9.2
- vLLM main:
cc876d0f29
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Signed-off-by: MengqingCao <cmq0113@163.com>