[Build] Force torch version (#3791)

We notice that sometimes user build vllm-ascend with incorrect torch
version. In this case, the build is passed, but when running the code,
the error `AttributeError: '_OpNamespace' '_C_ascend' object has no
attribute 'weak_ref_tensor'` is raised. Let's force the torch version to
2.7.1 and check the torch version when build from source to fix the
issue.

closes: #3342

- vLLM version: v0.11.0rc3
- vLLM main:
c9461e05a4

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

examples/disaggregated_prefill_v1/mooncake_connector_deployment_guide.md

@@ -5,7 +5,7 @@
  *  Software:
      *  Python >= 3.9, < 3.12
      *  CANN >= 8.2.rc1
-     *  PyTorch >= 2.7.1, torch-npu >= 2.7.1.dev20250724
+     *  PyTorch == 2.7.1, torch-npu == 2.7.1.dev20250724
      *  vLLM (same version as vllm-ascend)
      *  mooncake-transfer-engine reference documentation: https://github.com/kvcache-ai/Mooncake/blob/main/doc/zh/ascend_transport.md
 

examples/disaggregated_prefill_v1/mooncake_connector_store_deployment_guide.md

@@ -5,7 +5,7 @@
 * Software:
   * Python >= 3.9, < 3.12
   * CANN >= 8.2.rc1
-  * PyTorch >= 2.7.1, torch-npu >= 2.7.1.dev20250724
+  * PyTorch == 2.7.1, torch-npu == 2.7.1.dev20250724
   * vLLM：main branch
   * vLLM-Ascend：main branch
   * Mooncake：[AscendTransport/Mooncake at pooling-async-memcpy](https://github.com/AscendTransport/Mooncake/tree/pooling-async-memcpy)(Currently available branch code, continuously updated.)