Fix device issue in `OpenLlamaModelTest::test_model_parallelism`
# What does this PR do?

See the comments in the changes.

Currently, CI has a failure 

```bash
src/transformers/models/open_llama/modeling_open_llama.py:740: in forward
    logits = torch.einsum("blh,vh->blv", hidden_states, self.model.embed_tokens.weight)
   ...
   ...
   RuntimeError: Expected all tensors to be on the same device, but found at least two devices, cuda:1 and cuda:0! (when checking argument for argument mat2 in method wrapper_CUDA_bmm)
```
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