[Bugfix] Improve Triton stability on Ascend for large grids (#6301)

### What this PR does / why we need it?
Improve Triton stability on Ascend for large grids
set `TRITON_ALL_BLOCKS_PARALLEL=1` when grids > 65535

- vLLM version: v0.14.1
- vLLM main:
dc917cceb8

Signed-off-by: hfadzxy <starmoon_zhang@163.com>

vllm_ascend/ops/rotary_embedding.py

@@ -16,6 +16,7 @@
 #
 
 import math
+import os
 from typing import Optional, Tuple
 
 import torch
@@ -547,6 +548,9 @@ class AscendDeepseekScalingRotaryEmbedding(DeepseekScalingRotaryEmbedding):
 
 class AscendMRotaryEmbedding(MRotaryEmbedding):
 
+    # Empirical safety threshold for large Triton grids on Ascend NPU
+    _ASCEND_TRITON_GRID_LIMIT = 65535
+
     def forward_triton(self,
                        positions: torch.Tensor,
                        query: torch.Tensor,
@@ -568,6 +572,12 @@ class AscendMRotaryEmbedding(MRotaryEmbedding):
 
         assert self.mrope_section
 
+        # When the grid becomes large, enable TRITON_ALL_BLOCKS_PARALLEL 
+        # to avoid scheduler/runtime failures.
+        if (query_shape[0] > self._ASCEND_TRITON_GRID_LIMIT and 
+                os.environ.get("TRITON_ALL_BLOCKS_PARALLEL") != "1"):
+            os.environ["TRITON_ALL_BLOCKS_PARALLEL"] = "1"
+
         q, k = triton_mrope(
             query,
             key,