[ViewOpGraph] Improve GraphViz output (#125509)
This patch improves the GraphViz output of ViewOpGraph
(--view-op-graph).
- Switch to rectangular record-based nodes, inspired by a similar
visualization in [Glow](https://github.com/pytorch/glow). Rectangles
make more efficient use of space when printing text.
- Add input and output ports for each operand and result, and remove
edge labels.
- Switch to a muted color palette to reduce eye strain.
