Make nested TreeSpec printing nicer (#46538) (#86546)
1. Made TreeSpec into a dataclass.
2. In `__repr__`, recursively transformed TreeSpec into dictionaries and then pretty-printed it.
Fixes #46538. Hi, @ezyang. this PR is for the TreeSpec `__repr__` refactor we discussed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86546
Approved by: https://github.com/ezyang
