Add self-hosted GPU workflow in CI

[ijpulidos]

In order to run larger and more production-like tests we would need to include a self-hosted GHA workflow with GPU support for our CI. Similar to what is done in https://github.com/choderalab/perses

It was brought to our attention that https://salad.com/pricing could be a good option especially in terms of costs.

[mikemhenry]

I will wire up OpenFE's GPU CI infrastructure so we can run the tests

[mikemhenry]

@ijpulidos how time sensitive are we for the GPU CI? With how the GPU CI works, I will either need to transfer this repo into the OpenFE Org (which is the plan, but needs John's sign off) or make a fork of this repo in the OpenFE Org.

[ijpulidos]

@mikemhenry Let's wait for the stakeholders' approval! Hopefully this week we will get things migrated to the OpenFE org.