Make PartialModelMetaclass only modify BaseModel subclass

due to ModelMetaclass also being called in other situations where cls is
not BaseModel. We don't want this to interfere in those situations.

See https://github.com/pydantic/pydantic/blob/8128821b864db533b7c5c0c75589777c10d5cda7/pydantic/_internal/_model_construction.py#L85-L88

Author: kiendang

packages/syft/src/syft/types/syft_metaclass.py

@@ -38,10 +38,12 @@ def __new__(
     ) -> type:
         cls = super().__new__(mcs, cls_name, bases, namespace, *args, **kwargs)
 
-        for field_info in cls.model_fields.values():
-            if field_info.annotation is not None and field_info.is_required():
-                field_info.annotation = field_info.annotation | EmptyType
-                field_info.default = Empty
+        if issubclass(cls, BaseModel):
+            for field_info in cls.model_fields.values():
+                if field_info.annotation is not None and field_info.is_required():
+                    field_info.annotation = field_info.annotation | EmptyType
+                    field_info.default = Empty
+
+            cls.model_rebuild(force=True)
 
-        cls.model_rebuild(force=True)
         return cls