TorchExplorer 0.5.1: major refactors, i/o node tooltips and visualization, vega improvements

Vega spec version 3b

Last "alpha" release before 1.0.0. Version 0.5.0 package is screwed up so this is the replacement.

Release notes:

1. Major code refactoring, both structural and line-by-line to improve clarity and modularity
2. Input/output nodes have tooltips showing tensor type and shape. They can be dragged into the right panel to visualize.
3. Small Vega improvements: magnifying glass locates parent node, histogram title and grad/grad norm text separated, left-align panel headers
4. Confirm desired functionality for nn.Embedding module, expand test suite
5. Documentation improvements