Remove pmap special-casing in PjRt-IFRT executable creation

[emilyfertig]

Expose xla::ifrt::Client::GetDefaultDeviceAssignment so the correct executable_devices can be plumbed through from pxla.py and remove the code in xla::ifrt::PjRtLoadedExecutable::CreateInternal that detects if jit(pmap) was called and substitutes the executable devices.