[main] remove torch.cat and replace it by List[0] (#2153)

### What this PR does / why we need it?
torch_npu.npu_grouped_matmul:

https://www.hiascend.com/document/detail/zh/Pytorch/710/apiref/torchnpuCustomsapi/context/torch_npu-npu_grouped_matmul.md

According to the document, when `split_item` is 2 or 3,
`torch_npu.npu_grouped_matmul` will return a list which has one element.
Therefore, the `torch.cat` after `torch_npu.npu_grouped_matmul` is
unnecessary.

### Does this PR introduce _any_ user-facing change?
not involved

### How was this patch tested?
ut and e2e covered: `tests/ut/ops/test_fused_ops.py`,
`tests/e2e/singlecard/ops/test_fused_moe.py`

**performance**:
(qwen3 30B, 2k->20k)

base:
Total Token throughput (tok/s):          667.76 

remove cat:
Total Token throughput (tok/s):          680.82 


- vLLM version: v0.10.0
- vLLM main:
fa00c5d75b

Signed-off-by: huangxialu <huangxialu1@huawei.com>

tests/ut/ops/test_fused_ops.py

@@ -112,7 +112,7 @@ def mock_moe_env(mocker: MockerFixture):
                 torch.randn(16, 2)
         )), \
         patch("torch_npu.npu_grouped_matmul", return_value=(
-                (torch.randn(8, 2), torch.randn(8, 2))
+                [torch.randn(16, 2)]
         )), \
         patch("torch_npu.npu_swiglu", return_value=(
                 torch.randn(16, 2)