Turn on multiprocessing eval for nightly gpu tests

[stephenroller]

The nightly gpu tests still (mostly) use only one gpu. We have full support for multiprocessing eval, and we should be taking advantage of it.

My suggestion is to add a "use_multiprocessing" argument to testing_utils.eval_model and have it toggle between the two scripts appropriately

We probably will see a 25% speedup in the long gpu tests. Measure the improvement locally (which should be about 2x), report here, and push to report improvements in real CI.

Note this is pretty much only useful for generative models, but there are a large number of those: at least the unlikelihood, and dodeca models. Perhaps Eric's style tests. Possibly the controllable models too.

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[stephenroller]

Now that we have some data caching, I think this is actually much more impactful, @dianaglzrico