### What this PR does / why we need it?
Extracts repeated `attn_metadata[layer_name].decode` access into a
single variable to improve code readability and reduce redundancy.
Uses `getattr` with a default value to safely access the decode
attribute, making the code more defensive against potential attribute
errors.
### Does this PR introduce _any_ user-facing change?
None.
### How was this patch tested?
None.
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c
Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>
543f122101