feat: make cupy optional in install.py

cupy is now a best-effort install for NVIDIA users. Non-CUDA setups
(ROCm, CPU) skip cupy and use PyTorch fallback kernels instead.

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

install.py

@@ -8,44 +8,39 @@ def get_cupy_package():
     try:
         import torch
         if not torch.cuda.is_available():
-            print("[Tween] WARNING: CUDA not available. cupy requires CUDA.")
             return None
         cuda_version = torch.version.cuda
         if cuda_version is None:
-            print("[Tween] WARNING: PyTorch has no CUDA version info.")
             return None
         major = int(cuda_version.split(".")[0])
         cupy_pkg = f"cupy-cuda{major}x"
-        print(f"[Tween] Detected CUDA {cuda_version}, will use {cupy_pkg}")
         return cupy_pkg
-    except Exception as e:
-        print(f"[Tween] WARNING: Could not detect CUDA version: {e}")
+    except Exception:
         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)
-
+    # Install core requirements first
     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.")
+
 
 if __name__ == "__main__":
     install()