Make it possible to trigger XLA/slow tests via commit message. (#18345)

Make it possible to trigger XLA/slow tests via commit message. (#18345)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18345
ghimport-source-id: 9649d76bb194866859d62e6ba2a3a265c96ebba5

Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18345 Make it possible to trigger XLA/slow tests via commit message.**

Four variants are supported: `[xla ci] [ci xla] [xla test] [test xla]`; substitute
xla with slow for slow tests.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Differential Revision: D14584557

fbshipit-source-id: fcbfdfb28246823135bb3d3910baae073d16e81d