Refactor visualize dataset stats from DataNodeMetadata to DataNode

[ravi-kumar-pilla]

As we plan to move the stats information from Metadata panel to Flowchart, we should move the stats field to be part of DataNode instead of DataNodeMetadata.

Refactor steps -

1. Remove stats field from DataNodeMetadata class
2. Update the DataNodeMetadata call from router
3. Add the stats field inside the data access manager (add_datasets method)
4. Access the stats field via the DataNode object
5. Modify the unit tests accordingly

graph_node = GraphNode.create_data_node(
                dataset_name=dataset_name,
                layer=layer,
                tags=set(),
                dataset=obj,
                is_free_input=is_free_input,
                stats = self.get_stats_for_data_node(dataset_name)
            )