[Misc] Nit fix for disaggregated_prefill and ascend_forward_context (#2097)

we recently added disaggregated_prefill and ascend_forward_context
feature by
ba3dfbd59e
and
df0ec55162.
This PR fix some nit introduced by them to make the code clear.
1. drop `current_platform` usage. It'll lead unknown circular import
error in some case
2. update `set_ascend_forward_context` function to make the logic clear.
for example, remove V0 support in this function.
3. Remove useless `self.local_rank_across_dp` in worker
4. Remove `soc_info.py` to use `get_ascend_soc_version` instead.
 

- vLLM version: v0.10.0
- vLLM main:
02f82fe438

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

vllm_ascend/utils.py

@@ -479,7 +479,8 @@ def register_ascend_customop():
     _ASCEND_CUSTOMOP_IS_REIGISTERED = True
 
 
-# TODO(zzzzwwjj): It will be judged with _build_info afterwards.
+# TODO(zzzzwwjj): Currently there is no clear SOC_VERSION policy for A2 and A3 in CANN.
+# So we get the version dynamically. In the future, we should get the version info from _build_info like 310p does.
 class AscendSocVersion(Enum):
     A2 = 0
     A3 = 1