[BUG] Save Only First Rank in DP will crash, when async-scheduling is enabled

[matthewygf]

Describe the bug
When turned on Save Only First Rank in DP deployment, the application will crash.

To Reproduce

• Turns on save_only_first_rank
• Turns on async_scheduling
• Observe

Expected behavior

• No crash

Screenshots
If applicable, add screenshots to help explain your problem.

Platform & Software (please complete the following information):

• CANN Version: 8.5.0
• Ascend HDK Version: 25.5.0
• Hardware: 910C
• vLLM Version: v0.18.0

Additional context

• should be related to the recent PR Sharded Broadcast with Dual-Stream Pipeline for Reduced Peak NPU Memory #257