### What this PR does / why we need it?
Now `seq_lens` was not being reset correctly after each step due to
missing code that clears the sequence lengths. As a result, when
processing a smaller batch after a larger batch, the `seq_lens` from the
larger batch was still carried over. This caused the attention operator
to compute using an unnecessarily larger sequence length, leading to an
increased computation load and performance degradation.
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
- vLLM version: v0.13.0
- vLLM main:
d68209402d
Signed-off-by: ZYang6263 <zy626375@gmail.com>