[memory_viz] Unified viewer (#103565)
This replaces the invidual visualization routines in _memory_viz.py with
a single javascript application.
The javascript application can load pickled snapshot dumps directly using
drag/drop, requesting them via fetch, or by embedding them in a webpage.
The _memory_viz.py commands use the embedding approach.
We can also host MemoryViz.js on a webpage to use the drag/drop approach, e.g.
https://zdevito.github.io/assets/viz/
(eventually this should be hosted with the pytorch docs).
All views/multiple cuda devices are supported on one page.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103565
Approved by: https://github.com/eellison, https://github.com/albanD
