Return custom num addressable devices in PJRT_LoadedExecutable_AddressableDevices

[ajakovljevicTT]

Currently, we are hardcoding the number of addressable devices to be returned when PJRT_LoadedExecutable_AddressableDevices, to be able to support multichip, this has to be custom, with this information inferred from the StableHLO code, possibly via the mhlo.num_partitions = 2 attribute. This will require changes to the ExecutableImage::get_num_addressable_devices() function. It also might necessitate changing this part of the code that returns concrete addressable devices, but is currently not tested:

args->addressable_devices = const_cast<PJRT_Device **>(
        reinterpret_cast<PJRT_Device *const *>(addressable_devices.data()));