cleanup ascend config (#5296)

1. refresh additional config doc
2. move kv config logic to platform.
3. improve `dump_config` init logic and rename it to `dump_config_path`
this change is user impacted. dump_config is changed from dict to
string.
4. correct `enable_async_exponential` type
5. remove useless `chunked_prefill_for_mla`

- vLLM version: release/v0.13.0
- vLLM main:
ad32e3e19c

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

tests/e2e/nightly/models/test_deepseek_r1_0528_w8a8.py

@@ -76,10 +76,7 @@ async def test_models(model: str, mode: str) -> None:
         "PYTORCH_NPU_ALLOC_CONF": "expandable_segments:True"
     }
     speculative_config = {"num_speculative_tokens": 1, "method": "mtp"}
-    additional_config = {
-        "chunked_prefill_for_mla": True,
-        "enable_weight_nz_layout": True
-    }
+    additional_config = {"enable_weight_nz_layout": True}
     server_args = [
         "--quantization", "ascend", "--data-parallel-size", "2",
         "--tensor-parallel-size", "8", "--enable-expert-parallel", "--port",