Refactor gnn training (#253)

* minor upds

* refactor: training pipeline

* feat: find gcs image path

* feat: feature generation in trainer

* feat: validation sets in training

* bug: hgraph forward passes with missing edge types

* refactor: hgnn trainer

* feat: functional training pipeline

* bug: set validation data

* refactor: combined train engine and pipeline

* refactor: infernce pipeline, evaluation

* moved files

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Co-authored-by: anna-grim <anna.grim@alleninstitute.org>