### What this PR does / why we need it?
Fix pipeline parallel break for mla & sfa attention backend caused by a
magic number in metadata builder. The error report:
`AttributeError: 'PPMissingLayer' object has no attribute 'self_attn'`
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
This PR was tested with "mp" backend (PP2TP8 on an A3 node) as well as
"ray" backend (PP2TP8 on two A2 nodes).
- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0
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Signed-off-by: linfeng-yuan <1102311262@qq.com>
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