Re-enable type checking for distributed_c10d.py
Type checking for distributed_c10d.py was inadvertently turned off in issues that have accumulated since.
Note: the backwards compatibility linter does not like some of these changes. But they were incorrect before. This needs human verification, however.
#suppress-api-compatibility-check
Pull Request resolved: https://github.com/pytorch/pytorch/pull/115223
Approved by: https://github.com/wconstab