Add tests on FNO + DeepONet forward pass, gradients, and training

[ChrisRackauckas]

The tests don't seem to cover the use and training of the operators, just a few properties. It would be good to get a few integration tests.

[ba2tripleO]

Do these tests need to run on gpu?

[pzimbrod]

Yeah I think that makes sense at least for a subset of those, we also track that in a separate issue (#27).

Flux.jl also contains CUDA in the tests and since this package also exports layers and is based on Flux quite a bit, I would more or less stick to this.

[ChrisRackauckas]

I would start by making the tests on CPU though, then create equivalent tests on GPU. GPU will have a separate set of issues and it would be easier to catch/debug the CPU issues (usually correctness) first.