Fix the following beam error at pickling for dynamic config builder classes.

[copybara-service]

Fix the following beam error at pickling for dynamic config builder classes.
Top-level exception: Can't pickle <class 'google3.learning.brain.research.conversation.meena.data.dataset.generic_schema_based_tfds_proto_builder.load_builder_cls_from_file..ConfigBuilder'>: it's not found as google3.learning.brain.research.conversation.meena.data.dataset.generic_schema_based_tfds_proto_builder.load_builder_cls_from_file..ConfigBuilder

[google-cla]

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