change TCAV to use captum/_utils/fb/av.py

Summary:
AV.load now returns a Dataset, not a DataLoader
Also, activations for a single layer and multiple concepts are no longer stored as a single DataLoader (which also generates labels).  Instead, they are stored as a list of Datasets (not DataLoaders).  This list is used to create a Dataset (not DataLoader) that also generates labels.  I think it is better to pass around Datasets for as long as possible, because they are more flexible.

Differential Revision: D30566560

fbshipit-source-id: 4d3eaa648ddfb4beb60870c644268bd8e0dc6c9d

Author: 99warriors