### What this PR does / why we need it?
add rejection sampler ut.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
UT passed
- vLLM version: v0.10.0
- vLLM main:
586f286789
Signed-off-by: wangxiaoxin-sherie <wangxiaoxin7@huawei.com>