Need to run Realm e2e test on 2 GPUs to match mapping

[elliottslaughter]

#1651 unified some code to generate the MPCG for Realm e2e tests. That's fine, but results in the CUDA test assuming 2 GPUs available.

There are two ways to fix this:

1. Actually run on 2 GPUs
2. Make the MPCG configurable so we don't assume 2 devices

For now option (1) seems easier and if we're moving GPU CI to Sapling eventually anyway, having 2 GPUs is a trivial requirement to satisfy.

Makes all tests pass on master.

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This change is