[Doc][Example][Bugfix] Elements in local_device_ids should be casted … (#3782)

### What this PR does / why we need it?
It's a tiny bugfix in the `gen_ranktable.py` script. The script is an
util to help setup an example case. It is used to prepare a ranktable
before disaggregated prefill deployment.

Elements in `local_device_ids` list should be casted to `int` type
before referred for a MOD math operation.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
No.


- vLLM version: v0.11.0
- vLLM main:
vllm-project/vllm@c9461e0

---------

Signed-off-by: paulyu12 <[email protected]>

Author: paulyu12

examples/disaggregated_prefill_v1/gen_ranktable.py

@@ -63,7 +63,11 @@ def get_cmd_stdout(cmd):
 chips_per_card = int(chips_per_card)
 
 if args.local_device_ids:
-    local_device_ids = args.local_device_ids.split(',')
+    try:
+        local_device_ids = [int(id_str) for id_str in args.local_device_ids.split(',')]
+    except ValueError:
+        print(f"Error: --local-device-ids must be a comma-separated list of integers. Received: '{args.local_device_ids}'")
+        exit(1)
 else:
     local_device_ids = []
     for card_id in range(num_cards):