Fix gradient mask values range

Author: StAlKeR7779

invokeai/app/invocations/create_gradient_mask.py

@@ -93,6 +93,7 @@ def invoke(self, context: InvocationContext) -> GradientMaskOutput:
 
             # redistribute blur so that the original edges are 0 and blur outwards to 1
             blur_tensor = (blur_tensor - 0.5) * 2
+            blur_tensor[blur_tensor < 0] = 0.0
 
             threshold = 1 - self.minimum_denoise