Support training-only components in ComponentGraph

[angela97lin]

Right now, we support samplers as training-only components by checking if the component is a sampler component and not taking any action if so:

evalml/evalml/pipelines/component_graph.py

Line 323 in 4afe6dd

elif isinstance(component_instance, BaseSampler):

Once we add other components that are train-only (ex: drop rows), we should verify our behavior holds for other components by updating this line to be more general.

[dsherry]

One idea: tagging. All training-only components get static attribute TRAINING_ONLY, and we update this elif to check that static tag.