fix: make cupy installation opt-in

install.py

@@ -1,45 +1,20 @@
+import os
 import subprocess
 import sys
-import os
-
-
-def get_cupy_package():
-    """Detect PyTorch's CUDA version and return the matching cupy package name."""
-    try:
-        import torch
-        if not torch.cuda.is_available():
-            return None
-        cuda_version = torch.version.cuda
-        if cuda_version is None:
-            return None
-        major = int(cuda_version.split(".")[0])
-        cupy_pkg = f"cupy-cuda{major}x"
-        return cupy_pkg
-    except Exception:
-        return None
 
 
 def install():
-    # Install core requirements first
+    """Install required dependencies without mutating optional GPU packages."""
     requirements_path = os.path.join(os.path.dirname(__file__), "requirements.txt")
     subprocess.check_call([
         sys.executable, "-m", "pip", "install", "-r", requirements_path
     ])
-
-    # Try to install cupy for NVIDIA users (optional, improves performance)
-    cupy_pkg = get_cupy_package()
-    if cupy_pkg:
-        try:
-            subprocess.check_call([
-                sys.executable, "-m", "pip", "install", cupy_pkg
-            ])
-            print(f"[Tween] cupy installed ({cupy_pkg}) — fast CUDA kernels enabled")
-        except subprocess.CalledProcessError:
-            print(f"[Tween] WARNING: Could not install {cupy_pkg}. "
-                  f"BIM-VFI, SGM-VFI, and GIMM-VFI will use slower PyTorch fallback.")
-    else:
-        print("[Tween] cupy skipped (no NVIDIA CUDA). "
-              "BIM-VFI, SGM-VFI, and GIMM-VFI will use PyTorch fallback.")
+    print(
+        "[Tween] Optional cupy is not installed automatically. "
+        "BIM-VFI, SGM-VFI, and GIMM-VFI use the PyTorch fallback unless you "
+        "install the matching cupy wheel manually; EMA-VFI, SPEED, and "
+        "LDF-VFI do not use cupy."
+    )
 
 
 if __name__ == "__main__":