Add cupy check to Load nodes with install instructions

BIM-VFI, SGM-VFI, and GIMM-VFI Load nodes now check for cupy at
load time and raise a clear error with the user's CUDA version and
the exact pip install command. Updated README with step-by-step
cupy install instructions.

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

nodes.py

@@ -18,6 +18,28 @@ from .gimm_vfi_arch import clear_gimm_caches
 logger = logging.getLogger("Tween")
 
 
+def _check_cupy(model_name):
+    """Raise a clear error if cupy is not installed."""
+    try:
+        import cupy  # noqa: F401
+    except ImportError:
+        try:
+            cuda_ver = torch.version.cuda or "unknown"
+            major = int(cuda_ver.split(".")[0])
+            cupy_pkg = f"cupy-cuda{major}x"
+        except Exception:
+            cuda_ver = "unknown"
+            cupy_pkg = "cupy-cuda12x  # adjust to your CUDA version"
+        raise RuntimeError(
+            f"{model_name} requires cupy but it is not installed.\n\n"
+            f"Your PyTorch CUDA version: {cuda_ver}\n\n"
+            f"Install it with:\n"
+            f"  pip install {cupy_pkg}\n\n"
+            f"If you are unsure of your CUDA version, run:\n"
+            f"  python -c \"import torch; print(torch.version.cuda)\""
+        )
+
+
 def _compute_target_fps_params(source_fps, target_fps):
     """Compute oversampling parameters for target FPS mode.
 
@@ -138,6 +160,7 @@ class LoadBIMVFIModel:
     CATEGORY = "video/BIM-VFI"
 
     def load_model(self, model_path, auto_pyr_level, pyr_level):
+        _check_cupy("BIM-VFI")
         full_path = os.path.join(MODEL_DIR, model_path)
 
         if not os.path.exists(full_path):
@@ -1110,6 +1133,7 @@ class LoadSGMVFIModel:
     CATEGORY = "video/SGM-VFI"
 
     def load_model(self, model_path, tta, num_key_points):
+        _check_cupy("SGM-VFI")
         full_path = os.path.join(SGM_MODEL_DIR, model_path)
 
         if not os.path.exists(full_path):
@@ -1522,6 +1546,7 @@ class LoadGIMMVFIModel:
     CATEGORY = "video/GIMM-VFI"
 
     def load_model(self, model_path, ds_factor):
+        _check_cupy("GIMM-VFI")
         full_path = os.path.join(GIMM_MODEL_DIR, model_path)
 
         # Auto-download main model if missing