### What this PR does / why we need it?
Adds a validation check to prevent running with an incompatible
configuration.
The `ASCEND_LAUNCH_BLOCKING=1` environment variable, used for debugging,
enforces synchronous execution which is incompatible with ACL Graph.
This change raises an explicit error to inform the user about the
conflict and how to resolve it, preventing a more obscure failure later.
### Does this PR introduce _any_ user-facing change?
None.
### How was this patch tested?
None needed.
- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0
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Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>
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