Fix a bug where cached outputs affected IS_CHANGED (#4535)

This change fixes a bug where non-constant values could be passed to the
IS_CHANGED function. This would result in workflows taking an extra
execution before they acted as if they were cached.

The actual change is like 4 characters -- the rest is adding unit tests.

tests/inference/testing_nodes/testing-pack/stubs.py

@@ -28,6 +28,28 @@ class StubImage:
         elif content == "NOISE":
             return (torch.rand(batch_size, height, width, 3),)
 
+class StubConstantImage:
+    def __init__(self):
+        pass
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {
+                "value": ("FLOAT", {"default": 0.5, "min": 0.0, "max": 1.0, "step": 0.01}),
+                "height": ("INT", {"default": 512, "min": 1, "max": 1024 ** 3, "step": 1}),
+                "width": ("INT", {"default": 512, "min": 1, "max": 4096 ** 3, "step": 1}),
+                "batch_size": ("INT", {"default": 1, "min": 1, "max": 1024 ** 3, "step": 1}),
+            },
+        }
+
+    RETURN_TYPES = ("IMAGE",)
+    FUNCTION = "stub_constant_image"
+
+    CATEGORY = "Testing/Stub Nodes"
+
+    def stub_constant_image(self, value, height, width, batch_size):
+        return (torch.ones(batch_size, height, width, 3) * value,)
+
 class StubMask:
     def __init__(self):
         pass
@@ -93,12 +115,14 @@ class StubFloat:
 
 TEST_STUB_NODE_CLASS_MAPPINGS = {
     "StubImage": StubImage,
+    "StubConstantImage": StubConstantImage,
     "StubMask": StubMask,
     "StubInt": StubInt,
     "StubFloat": StubFloat,
 }
 TEST_STUB_NODE_DISPLAY_NAME_MAPPINGS = {
     "StubImage": "Stub Image",
+    "StubConstantImage": "Stub Constant Image",
     "StubMask": "Stub Mask",
     "StubInt": "Stub Int",
     "StubFloat": "Stub Float",