### What this PR does / why we need it?
MLA and GQA use different computation logic: MLA slice batches and only
compute on the actually valid tokens. That means outer padding must be
handled carefully — the accuracy issue this PR fixes was caused by stale
data in `slot_mapping` being reused by subsequent inference steps.
So we zeros out the portion of the slot mapping tensor that is not used
by the currently scheduled tokens.
### Does this PR introduce _any_ user-facing change?
None.
### How was this patch tested?
Working on it.
- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0
Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>
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