[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>

pyproject.toml

@@ -15,7 +15,7 @@ requires = [
     "setuptools>=64",
     "setuptools-scm>=8",
     "torch-npu==2.7.1.dev20250724",
-    "torch>=2.7.1",
+    "torch==2.7.1",
     "torchvision",
     "wheel",
     "msgpack",