Jax doesn't default to cpu when cuda is not available

[thomasgaudelet]

Hi, I have a general requirement file that I use across devices, including for unit testing on github, but since latest version of jax, it is crashing when there are no gpu available with the error:

RuntimeError: Unable to initialize backend 'cuda': FAILED_PRECONDITION: No visible GPU devices. (you may need to uninstall the failing plugin package, or set JAX_PLATFORMS=cpu to skip this backend.)

It seems that the package doesn't default to using cpu now when there's no gpu available. Is it intended? It seems linked to the jax-cuda12-plugin extra requirements as I didn't have this issue with 0.4.28 release and ["cuda_pip"].