[Perf] move quant before allgather in Allgather EP (#3420)

### What this PR does / why we need it?
move quant before allgather in Allgather EP, rely on
https://github.com/vllm-project/vllm-ascend/pull/3334

Deepseek R1 W8A8 performance on A2 with
`HCCL_ALGO="level0:NA;level1:pipeline"`:
| Seq length | Mean TTFT (ms) main | Mean TTFT (ms)  this PR |
|----------|----------|----------|
| 4k   |  375.21  | 364.99   |
| 16k  | 1465.23   | 1421.75  |
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?


- vLLM version: v0.11.0
- vLLM main:
83f478bb19

---------

Signed-off-by: realliujiaxu <realliujiaxu@163.com>

vllm_ascend/ops/register_custom_ops.py

@@ -36,7 +36,7 @@ def _maybe_all_gather_and_maybe_unpad_impl(
             x = tensor_model_parallel_all_gather(x, 0)
             pad_size = forward_context.pad_size
             if pad_size > 0:
-                x = x[:-pad_size, :]
+                x = x[:-pad_size]
         else:
             x = get_ep_group().all_gather(x, 0)
             # unpad
@@ -50,8 +50,7 @@ def _maybe_all_gather_and_maybe_unpad_impl(
             offset = 0
             for idx in range(dp_size):
                 num_tokens_dp = num_tokens_across_dp_cpu[idx]
-                result[offset:offset +
-                       num_tokens_dp, :] = x[idx, :num_tokens_dp, :]
+                result[offset:offset + num_tokens_dp] = x[idx, :num_tokens_dp]
                 offset += num_tokens_dp
             x = result