[bugfix] Fix Qwen3-30B-A3B dp parallel hung issue when running with the dp parallel example (#3287)

### What this PR does / why we need it?
Fix Qwen3-30B-A3B dp parallel hung issue when running with the dp
parallel example.
For large-parameter models of Qwen3-30B and above, weight loading alone
takes 4 to 5 minutes. Therefore, the 5-minute timeout in the current
example code implementation is too short, causing some DP instances to
be killed prematurely and eventually stuck in the DP synchronization
all-reduce operation.

### Does this PR introduce _any_ user-facing change?
NA

### How was this patch tested?
NA

vLLM version: v0.11.0rc3
vLLM main: vllm-project/vllm@releases/v0.11.0

- vLLM version: v0.11.0rc3
- vLLM main:
vllm-project/vllm@releases/v0.11.0

---------

Signed-off-by: leo-pony <[email protected]>

Author: leo-pony

examples/offline_data_parallel.py

@@ -244,10 +244,10 @@ def start(rank):
         procs.append(proc)
     exit_code = 0
     for proc in procs:
-        proc.join(timeout=300)
+        proc.join(timeout=900)
         if proc.exitcode is None:
             print(
-                f"Killing process {proc.pid} that didn't stop within 5 minutes."
+                f"Killing process {proc.pid} that didn't stop within 15 minutes."
             )
             proc.kill()
             exit_code = 1