Fix str + repr of multi-slice Devices

[skye]

I think multi-slice TPU devices should still be called TpuDevice instead of MegaScalePjRtDevice. You can already tell the device is part of a multi-slice config because it has a slice_id. It's very verbose right now:

In [2]: d = jax.devices()[0]

In [3]: str(d)
Out[3]: 'MegaScalePjRtDevice(wrapped=TPU_0(process=0,(0,0,0,0)), slice_id=0)'

In [4]: repr(d)
Out[4]: 'MegaScalePjRtDevice(wrapped=TpuDevice(id=0, process_index=0, coords=(0,0,0), core_on_chip=0), slice_id=0)'