[refactor] Refactor the interface for shard weight and remove the flashcomm2 o_shared interface. (#5181)

### What this PR does / why we need it?
- Delete the environment variable
`VLLM_ASCEND_ENABLE_FLASHCOMM2_OSHARED`
- Introduce layer_sharding as a configurable feature in
additional_config
- Revise the term "shared weight" to "shard weight."
Configuration : The feature is opt-in via the additional_config
argument:
```
--additional-config '{
  "layer_sharding": ["o_proj", "q_b_proj"]
}'
```

This is orthogonal to standard tensor parallelism and weight replication
strategies. It is treated as a separate, explicit feature.It can be used
in any scenario, combined with the
flashcomm2https://github.com/vllm-project/vllm-ascend/pull/3232 feature
or the ShardedCP #4702 feature, to achieve significant performance.



- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: zzhx1 <zzh_201018@outlook.com>
Signed-off-by: zzhxx <zhangzihang23@mails.ucas.ac.cn>
Signed-off-by: chenxiao <Jaychou1620@Gmail.com>
Co-authored-by: clrs97 <524936896@qq.com>
Co-authored-by: Levi-JQ <yujinqi2@huawei.com>
Co-authored-by: chenxiao <Jaychou1620@Gmail.com>

docs/source/user_guide/configuration/additional_config.md

@@ -49,6 +49,7 @@ The following table lists additional configuration options available in vLLM Asc
 | `expert_map_record_path`            | str  | `None`  | Save the expert load calculation results to a new expert table in the specified directory.                |
 | `init_redundancy_expert`            | int  | `0`     | Specify redundant experts during initialization.                                                          |
 | `enable_kv_nz`                      | bool | `False` | Whether to enable kvcache NZ layout. This option only takes effects on models using MLA (e.g., DeepSeek).                                      |
+| `layer_sharding` | dict | `{}` | Configuration options for layer sharding linear |
 
 The details of each configuration option are as follows: