Make `torch.nn` importable on Python-3.7.0
As `typing.OrderedDict` were introduced by Python-3.7.2+, see
https://docs.python.org/3.10/library/typing.html#typing.OrderedDict
Fixes #74087
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74211
Approved by: https://github.com/suo, https://github.com/janeyx99
