enable online serving quantization (#877)

For online serving, "ascend" quantization method is not a choice
natively, so we need to add "ascend" quantization method to quantization
methods list and the user can enable quantization using "vllm serve
--quantization ascend" command.

---------

Signed-off-by: 22dimensions <waitingwind@foxmail.com>

vllm_ascend/utils.py

@@ -38,6 +38,8 @@ else:
 # Maximum number of graphs that can be captured by ACL Graph
 MAX_CAPTURE_SIZE = 1920
 
+ASCEND_QUATIZATION_METHOD = "ascend"
+
 
 def try_register_lib(lib_name: str, lib_info: str = ""):
     import importlib