[Fix] Cap max tokens to prevent potential OOM (#3720)

### What this PR does / why we need it?
Caps the calculated maximum number of tokens at 512.

This prevents allocating an excessively large buffer when a cudagraph
capture size is not specified, mitigating the risk of out-of-memory
errors.

### Does this PR introduce _any_ user-facing change?
None.

### How was this patch tested?
None.

- vLLM version: v0.11.0rc3
- vLLM main:
17c540a993

Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>

vllm_ascend/worker/model_runner_v1.py

@@ -586,7 +586,9 @@ class NPUModelRunner(LoRAModelRunnerMixin):
         if self.compilation_config.cudagraph_capture_sizes:
             max_num_tokens = self.compilation_config.cudagraph_capture_sizes[0]
         else:
-            max_num_tokens = self.max_num_reqs * self.uniform_decode_query_len
+            # NOTE: To save memory, we cap the max number of tokens to 512.
+            max_num_tokens = min(
+                self.max_num_reqs * self.uniform_decode_query_len, 512)
         tp_size = self.parallel_config.tensor_parallel_size
         # Use integer arithmetic for ceiling division.
         num_tokens_per_tp_rank = (max_num_tokens + tp_size - 1) // tp_size