`model.tie_weights()` should be applied after `accelerator.prepare()` (#18676)
* `model.tie_weights()` should be applied after `accelerator.prepare`
Weight tying should be done after the model has been moved to XLA device as mentioned on PyTorch/XLA Troubleshooting guide [here](https://github.com/pytorch/xla/blob/master/TROUBLESHOOTING.md#xla-tensor-quirks)
* format code