Revive static_runtime_benchmark build and test (#87660)
This build uses the wrong BUILD_ENVIRONMENT `pytorch-linux-focal-py3`, thus it hasn't been run for a long time (forgotten). The name was probably the old name of the build environment we used in the past. The convention today doesn't have the `pytorch-` prefix. There is a TODO for this:
> TODO: this condition is never (BUILD_ENVIRONMENT doesn't start with pytorch-), need to fix this.
This is done as part of [T131829540](https://www.internalfb.com/intern/tasks/?t=131829540), where we want
`static_runtime_benchmark` build and test jobs to run in OSS CI to avoid breaking internal
* I also fix some compiler warning errors `-Werror=sign-compare`, `-Werror,-Wunused-const-variable`, and gcc7 compatibility issue along the way because this hasn't been run for a long time.
* Reviving this test also reveals a small bug in `PrepackWeights` test in `test_static_runtime.cc` added recently in https://github.com/pytorch/pytorch/pull/85289. The test refers to an internal ops and should only be run internally. This has been fixed by https://github.com/pytorch/pytorch/pull/87799 (To be merged)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87660
Approved by: https://github.com/malfet
