tidy(nodes): move all field things to fields.py

Unfortunately, this is necessary to prevent circular imports at runtime.

tests/aa_nodes/test_nodes.py

@@ -3,12 +3,11 @@ from typing import Any, Callable, Union
 from invokeai.app.invocations.baseinvocation import (
     BaseInvocation,
     BaseInvocationOutput,
-    InputField,
     InvocationContext,
-    OutputField,
     invocation,
     invocation_output,
 )
+from invokeai.app.invocations.fields import InputField, OutputField
 from invokeai.app.invocations.image import ImageField