Add SkyPilot example for running benchmarks

[Michaelvll]

This PR is to add the SkyPilot example for the FlexGen benchmark. It will make the benchmark and setup more reproducible and convenient to manage.

Note the current yaml can only be run with the latest master branch of SkyPilot. It will be available for the pypi version soon later this week after we release the patch version.

Several future TODOs for SkyPilot:

1. Add the support for changing the disk type to be used for the instance so that we can run the commands that require high-performance SSD disks.

Tested:

• sky launch -c flexgen --use-spot --detach-setup ./flexgen/apps/task.yaml