[RMP] Subgraph capabilities across Merlin

[EvenOldridge]

Enable the division and retrieval of sub portions of the graph. Incorporate Subgraph usage throughout all of the merlin framework. The multi stage example needs to be refactored to correctly use subgraph workflows.

• Update the multi-stage example to include NVTabular workflows for user and item towers

  • Update feast feature store values to use non processed values
  • create subgraphs for nvtabular workflow for feature processing in notebook 1
  • load workflow from notebook 1 into notebook 2 and pull subgraphs from workflow
  • use subgraph workflows to process features in systems graph in notebook 2

• Create Support for usage of subgraphs in NVTabular

• Create support for usage of subgraphs in Systems

• Ensure loading and execution of full notebooks with subgraphs

• Update the text of the notebook to explain what's happening

• Update the system diagram in the notebook to show where NVT workflows are executed

[jperez999]

#1022

[karlhigley]

I don't think this is actually done, since the notebook still needs updates to the text and diagrams. I've added additional tasks to track that work.