Write detected cupy variant into requirements.txt at install time

install.py now updates requirements.txt with the correct cupy-cuda
package matching PyTorch's CUDA version before running pip install.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

install.py

@@ -14,8 +14,7 @@ def get_cupy_package():
         if cuda_version is None:
             print("[BIM-VFI] WARNING: PyTorch has no CUDA version info.")
             return None
-        major = cuda_version.split(".")[0]
-        major = int(major)
+        major = int(cuda_version.split(".")[0])
         cupy_pkg = f"cupy-cuda{major}x"
         print(f"[BIM-VFI] Detected CUDA {cuda_version}, will use {cupy_pkg}")
         return cupy_pkg
@@ -24,24 +23,29 @@ def get_cupy_package():
         return None
 
 
+def update_requirements(cupy_pkg):
+    """Write the correct cupy package into requirements.txt."""
+    requirements_path = os.path.join(os.path.dirname(__file__), "requirements.txt")
+    lines = []
+    if os.path.exists(requirements_path):
+        with open(requirements_path, "r") as f:
+            lines = [l.rstrip() for l in f if not l.strip().startswith("cupy")]
+    if cupy_pkg and cupy_pkg not in lines:
+        lines.append(cupy_pkg)
+    with open(requirements_path, "w") as f:
+        f.write("\n".join(lines) + "\n")
+
+
 def install():
+    cupy_pkg = get_cupy_package()
+    if cupy_pkg:
+        update_requirements(cupy_pkg)
+
     requirements_path = os.path.join(os.path.dirname(__file__), "requirements.txt")
     subprocess.check_call([
         sys.executable, "-m", "pip", "install", "-r", requirements_path
     ])
 
-    # Install cupy matching the current CUDA version
-    try:
-        import cupy
-        print("[BIM-VFI] cupy already installed, skipping.")
-    except ImportError:
-        cupy_pkg = get_cupy_package()
-        if cupy_pkg:
-            print(f"[BIM-VFI] Installing {cupy_pkg} to match PyTorch CUDA...")
-            subprocess.check_call([
-                sys.executable, "-m", "pip", "install", cupy_pkg
-            ])
-
 
 if __name__ == "__main__":
     install()