Fix: lazy import cupy to allow node loading without cupy installed

cupy was imported at module level, causing ComfyUI to fail loading
the node entirely if cupy wasn't installed yet. Now deferred to
first actual CUDA kernel call, with a clear error message.

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

bim_vfi_arch/costvol.py

@@ -1,12 +1,26 @@
 #!/usr/bin/env python
 
 import collections
-import cupy
 import os
 import re
 import torch
 import typing
 
+cupy = None
+
+def _ensure_cupy():
+    global cupy
+    if cupy is None:
+        try:
+            import cupy as _cupy
+            cupy = _cupy
+        except ImportError:
+            raise RuntimeError(
+                "cupy is required for BIM-VFI. Install it with:\n"
+                "  pip install cupy-cuda12x  (or cupy-cuda11x for CUDA 11)\n"
+                "Or run install.py from the Comfyui-BIM-VFI directory."
+            )
+
 
 ##########################################################
 
@@ -15,16 +29,19 @@ objCudacache = {}
 
 
 def cuda_int32(intIn:int):
+    _ensure_cupy()
     return cupy.int32(intIn)
 # end
 
 
 def cuda_float32(fltIn:float):
+    _ensure_cupy()
     return cupy.float32(fltIn)
 # end
 
 
 def cuda_kernel(strFunction:str, strKernel:str, objVariables:typing.Dict):
+    _ensure_cupy()
     if 'device' not in objCudacache:
         objCudacache['device'] = torch.cuda.get_device_name()
     # end
@@ -216,13 +233,16 @@ def cuda_kernel(strFunction:str, strKernel:str, objVariables:typing.Dict):
 # end
 
 
-@cupy.memoize(for_each_device=True)
+_cuda_launch_cache = {}
+
 def cuda_launch(strKey:str):
+    _ensure_cupy()
+    if strKey not in _cuda_launch_cache:
         if 'CUDA_HOME' not in os.environ:
             os.environ['CUDA_HOME'] = '/usr/local/cuda/'
         # end
-
-    return cupy.RawModule(code=objCudacache[strKey]['strKernel']).get_function(objCudacache[strKey]['strFunction'])
+        _cuda_launch_cache[strKey] = cupy.RawModule(code=objCudacache[strKey]['strKernel']).get_function(objCudacache[strKey]['strFunction'])
+    return _cuda_launch_cache[strKey]
 # end