Support running Opt on non default cudaStream about opt HOT 1 OPEN

Opt does not currently support non-default cudaStreams. There are three levels of support we could offer:

1. Internally create and use a non-default stream internally for all operations.
2. Expose the stream parameter for reading via the C API,
3. Expose the stream parameter for writing via the C API (probably in Opt_InitParams).

2 and 3 worry me because we'd need a portable way to expose the stream in the C API, which currently includes no headers, especially not cuda headers. We could probably be fine passing streams as (void*) though.

The first option is easier, and likely sufficient for your purposes. This will require non-trivial, but straightforward changes to API/src/solverGPUGaussNewton.t and API/src/util.t to use a stream when possible. This includes making many cudaMemcpy's into async to avoid unneccessary synchronization.

from opt.