[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/feature_guide/index.md

@@ -19,6 +19,7 @@ external_dp
 large_scale_ep
 ucm_deployment
 Fine_grained_TP
+layer_sharding
 speculative_decoding
 context_parallel
 :::

docs/source/user_guide/feature_guide/layer_sharding.md

@@ -0,0 +1,73 @@
+---
+title: Layer Sharding Guide
+---
+
+# Overview
+
+**Layer Shard Linear** is a memory-optimization feature designed for large language model (LLM) inference. It addresses the high memory pressure caused by **repeated linear operators across many layers** that share identical structure but have distinct weights.
+
+Instead of replicating all weights on every device, **Layer Shard Linear shards the weights of a "series" of such operators across the NPU devices in a communication group**:
+- The **i-th layer's linear weight** is stored **only on device `i % K`**, where `K` is the number of devices in the group.
+- Other devices hold a lightweight **shared dummy tensor** during initialization and fetch the real weight **on-demand via asynchronous broadcast** during the forward pass.
+
+As illustrated in the figure below, this design enables broadcast to reach weights: while the current layer (e.g., MLA or MOE) is being computed, the system **asynchronously broadcasts the next layer's weight** in the background. Because the attention computation in the MLA module is sufficiently latency-bound, the weight transfer for `o_proj` is **fully overlapped with computation**, making the communication **latency-free from the perspective of end-to-end inference**.
+
+This approach **preserves exact computational semantics** while **significantly reducing NPU memory footprint**, especially critical for:
+- Extremely deep architectures (e.g., DeepSeek-V3/R1 with 61 layers);
+- Models using **[DSA-CP](https://github.com/vllm-project/vllm-ascend/pull/4702)** or **[FlashComm2](https://github.com/vllm-project/vllm-ascend/pull/4188)**, where the full `O` (output) projection matrix must reside in memory per layer;
+- Scenarios where **attention computation latency fully overlaps** (hides) the communication cost of weight broadcasting.
+
+---
+
+## Flowchart
+![layer shard](./images/layer_sharding.png)
+
+> **Figure.** Layer Shard Linear workflow: weights are sharded by layer across devices (top), and during forward execution (bottom), asynchronous broadcast pre-fetches the next layer's weight while the current layer computes—enabling zero-overhead weight loading.
+
+---
+
+# Getting Started
+
+To enable **Layer Shard Linear**, specify the target linear layers using the `--additional-config` argument when launching your inference job. For example, to shard the `o_proj` and `q_b_proj` layers, use:
+
+```bash
+--additional-config '{
+  "layer_sharding": ["o_proj", "q_b_proj"]
+}'
+```
+
+---
+
+# Supported Scenarios
+
+This feature can be enabled in any scenario, but delivers the greatest benefit in the following cases:
+
+## FlashComm2-enabled
+
+When using [FlashComm2](https://github.com/vllm-project/vllm-ascend/pull/4188), the full output projection (`o_proj`) matrix must be resident in memory for each layer. Layer sharding significantly reduces memory pressure by distributing these weights across devices.
+
+**Example configuration:**
+
+```bash
+export VLLM_ASCEND_FLASHCOMM2_PARALLEL_SIZE=1
+vllm serve \
+  --model DeepSeek-V3/R1 \
+  --additional-config '{
+    "layer_sharding": ["o_proj"]
+  }'
+```
+
+## DSA-CP-enabled
+
+With [DSA-CP](https://github.com/vllm-project/vllm-ascend/pull/4702), both `q_b_proj` and `o_proj` layers require large weight matrices to be stored per layer. Sharding these layers across NPUs helps fit extremely deep models (e.g., 61-layer architectures) into limited device memory.
+
+**Example configuration:**
+
+```bash
+export VLLM_ASCEND_ENABLE_FLASHCOMM1=1
+vllm serve \
+  --model DeepSeek-V3.2 \
+  --additional-config '{
+    "layer_sharding": ["q_b_proj", "o_proj"]
+  }'
+```