Enable running of slow tests in CI. (#18236)

Enable running of slow tests in CI. (#18236)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18236
ghimport-source-id: 2bb80d017c2ea833669a2d55b340a922b2d44685

Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18236 Enable running of slow tests in CI.**
* #18231 Add a decorator for marking slow tests.

These tests only run on master, as they are slow.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Differential Revision: D14563115

fbshipit-source-id: f54ddef4abedc7e872e58657fc9ac537952773d0