Aggregate attention visualizations

[catherinesyeh]

Add aggregate attention visualizations for each head (e.g., try heatmap / Jesse Vig bipartite graphs)

• one resource: d3 heatmap

• [x] compute aggregate matrices

• [ ] load data into tool

• [ ] create heatmaps

Note: right now, probably mostly for single view, but potentially could add to matrix view too maybe?

[catherinesyeh]

done!