Fix create_cuda_kernel to allow CUSTOM_FIELDS to have a different size

The CPU backend allows Fields with FieldType.CUSTOM to have a differnt spatial_shape. The GPU backend should also.

pystencils/gpucuda/kernelcreation.py

@@ -20,8 +20,7 @@ def create_cuda_kernel(assignments,
     all_fields = fields_read.union(fields_written)
     read_only_fields = set([f.name for f in fields_read - fields_written])
 
-    buffers = set([f for f in all_fields if FieldType.is_buffer(f)])
-    fields_without_buffers = all_fields - buffers
+    buffers = set([f for f in all_fields if FieldType.is_buffer(f) or FieldType.is_custom(f)])
 
     field_accesses = set()
     num_buffer_accesses = 0
@@ -29,6 +28,7 @@ def create_cuda_kernel(assignments,
         field_accesses.update(eq.atoms(Field.Access))
         field_accesses = {e for e in field_accesses if not e.is_absolute_access}
         num_buffer_accesses += sum(1 for access in eq.atoms(Field.Access) if FieldType.is_buffer(access.field))
+    fields_without_buffers = all_fields - buffers
 
     common_shape = get_common_shape(fields_without_buffers)