Add examples to use multiple GPUs on HPC

[ichxw]

The current tutorial can only use one GPU. Could you add one example of how to use all available GPUs?

[IAlibay]

@ichxw at this moment OpenFE doesn't allow for a single transformation to parallelize over multiple GPUs.

The intended approach, for now, is that folks will run each of their transformation on a different GPU (you can see an example of this here https://github.com/OpenFreeEnergy/ExampleNotebooks/blob/main/rbfe_tutorial/cli_tutorial.md#running-the-simulations where you can submit each execution to a different job on a cluster).