Rename constants; fix shapes; add dtype (#1443)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1443

Rename constant nodes so they are easier to parse. Visualize shapes and dtype under "Attributes" (upper right corner when an op is selected). Shapes is also shown on edges.

Reviewed By: tarun292

Differential Revision: D52263486

fbshipit-source-id: 71b7b0c25d869b5a5f3ba7cefc194b598b662025