[PYTHON] GetItem not working for Client generated allOf model and broken since 5.2.0

Created by: the-akhil-nair

Problem: The issue is due to the creation of self._var_name_to_model_instances while doing the deserialization of the data.

Earlier the Python SDK code was using get_var_name_to_model_instances function which was adding var name to model instances that contain it.

Now the var_name_to_model_instances is populated with self and composed_instance and there will be no check if a variable name is present in composed_instances or not.

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Fixes https://github.com/OpenAPITools/openapi-generator/issues/11299