Default enable MLAPO (#5952)

### What this PR does / why we need it?
1) Default enable MLAPO for deepseek MLA Attention W8A8 models on PD
disagregation D Instance, for example: DeepSeekV3-W8A8,
DeepSeek-R1-W8A8.
2) Default enable MLAPO for DeepSeek SFA Attention W8A8 models,
currently is DeepSeek-V3.2-W8A8.

### Does this PR introduce _any_ user-facing change?
Don't need use manully to VLLM_ASCEND_ENABLE_MLAPO=1, to enable MLAPO
feature for deepseek w8a8 model

The effect of enabling MLAPO SFA model deployed on a single A3 Node:
Test
with:tests/e2e/nightly/single_node/models/test_deepseek_v3_2_exp_w8a8.py
dataset: gsm8k-lite，without set MTP, FULL GRAPH, has 19% promote：
未默认开启 MLAPO 时：
├─────────────────────────┤
│                TTFT                      │ 14055.8836 ms   │
├─────────────────────────┤
│                ITL                         │ 66.8171 ms.          │
├─────────────────────────┤
│ Output Token Throughput  │ 104.9105 token/s │
├─────────────────────────┤
默认开启 MLAPO 时：
├─────────────────────────┤
│                TTFT                      │ 3753.1547 ms   │
├─────────────────────────┤
│                ITL.                        │ 61.4236  ms.       │
├─────────────────────────┤
│ Output Token Throughput  │ 125.2075 token/s│
├─────────────────────────┤

- vLLM version: v0.13.0
- vLLM main:
2c24bc6996

---------

Signed-off-by: leo-pony <nengjunma@outlook.com>

vllm_ascend/envs.py

@@ -92,7 +92,11 @@ env_variables: dict[str, Callable[[], Any]] = {
     ),
     # Whether to enable msMonitor tool to monitor the performance of vllm-ascend.
     "MSMONITOR_USE_DAEMON": lambda: bool(int(os.getenv("MSMONITOR_USE_DAEMON", "0"))),
-    "VLLM_ASCEND_ENABLE_MLAPO": lambda: bool(int(os.getenv("VLLM_ASCEND_ENABLE_MLAPO", "0"))),
+    # Whether to enable MLAPO optimization for DeepSeek W8A8 series models.
+    # This option is enabled by default. MLAPO can improve performance, but
+    # it will consume more NPU memory. If reducing NPU memory usage is a higher priority
+    # for your DeepSeek W8A8 scene, then disable it.
+    "VLLM_ASCEND_ENABLE_MLAPO": lambda: bool(int(os.getenv("VLLM_ASCEND_ENABLE_MLAPO", "1"))),
     # Whether to enable weight cast format to FRACTAL_NZ.
     # 0: close nz;
     # 1: only quant case enable nz;