hotfix: make aot wheel work without nvcc (#1782)

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## 📌 Description

Change the logic of `get_cuda_version`: when nvcc is not installed,
return torch.version.cuda, this patch could unblock #1781 .

## 🔍 Related Issues

#1781 

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## 🧪 Tests

- [ ] Tests have been added or updated as needed.
- [ ] All tests are passing (`unittest`, etc.).

## Reviewer Notes
cc @Flynn-Zh

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: yzh119

flashinfer/jit/cpp_ext.py

@@ -55,13 +55,23 @@ def get_cuda_version() -> Version:
         nvcc = "nvcc"
     else:
         nvcc = os.path.join(CUDA_HOME, "bin/nvcc")
-    txt = subprocess.check_output([nvcc, "--version"], text=True)
-    matches = re.findall(r"release (\d+\.\d+),", txt)
-    if not matches:
-        raise RuntimeError(
-            f"Could not parse CUDA version from nvcc --version output: {txt}"
-        )
-    return Version(matches[0])
+    # Try to query nvcc for CUDA version; if nvcc is unavailable, fall back to torch.version.cuda
+    try:
+        txt = subprocess.check_output([nvcc, "--version"], text=True)
+        matches = re.findall(r"release (\d+\.\d+),", txt)
+        if not matches:
+            raise RuntimeError(
+                f"Could not parse CUDA version from nvcc --version output: {txt}"
+            )
+        return Version(matches[0])
+    except (FileNotFoundError, subprocess.CalledProcessError) as e:
+        # NOTE(Zihao): when nvcc is unavailable, fall back to torch.version.cuda
+        if torch.version.cuda is None:
+            raise RuntimeError(
+                "nvcc not found and PyTorch is not built with CUDA support. "
+                "Could not determine CUDA version."
+            ) from e
+        return Version(torch.version.cuda)
 
 
 def is_cuda_version_at_least(version_str: str) -> bool: