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/singlecard/test_sampler.py

@@ -62,6 +62,6 @@ def test_qwen3_exponential_overlap() -> None:
                     max_model_len=8192,
                     gpu_memory_utilization=0.7,
                     additional_config={
-                        "enable_async_exponential": 1,
+                        "enable_async_exponential": True,
                     }) as runner:
         runner.generate(example_prompts, sampling_params)