support pangumoe w8a8c8 and docs (#1477)

### What this PR does / why we need it?
support pangu moe w8a8c8

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
CI passed with new added test.

Signed-off-by: zhuyilin <809721801@qq.com>

vllm_ascend/platform.py

@@ -124,6 +124,10 @@ class NPUPlatform(Platform):
         model_config = vllm_config.model_config
         parallel_config = vllm_config.parallel_config
         cache_config = vllm_config.cache_config
+        kv_cache_dtype = vllm_config.additional_config.get(
+            "kv_cache_dtype", None)
+        if kv_cache_dtype is not None:
+            vllm_config.cache_config.cache_dtype = kv_cache_dtype
 
         if parallel_config:
             # Default value for expert tensor parallel size