Implement named tensor `unflatten(dim, namedshape)`. (#25658)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25658
This unflattens `dim` according to the shape specified in `namedshape`.
`namedshape` may be either an OrderedDict or an iterable of (name, size)
tuples.
Future:
- It is possible to make it take a dict in Python >= 3.6 because those are
ordered by default, but I'll leave that task for the future.
Test Plan: - new tests [namedtensor ci]
Differential Revision: D17192655
Pulled By: zou3519
fbshipit-source-id: fd9bd2f462c23a4df1c23d66f2aa95076ff1b160
