Fix duplicate code at T5Model
# What does this PR do?

Unlike `T5ForConditionalGeneration`, it doesn't seem to be necessary at `T5Model`.

@patrickvonplaten

_The documentation is not available anymore as the PR was closed or merged._<|||||>Hey @lkm2835,

could you give a bit more feedback on why this is not necessary? I don't exactly see why this is necessary at the moment.<|||||>Oh, sorry @patrickvonplaten 

1411-1412 and 1414-1415 are same code.
```
1411        if self.model_parallel:
1412            torch.cuda.set_device(self.decoder.first_device)
1413        # Set device for model parallelism
1414        if self.model_parallel:
1415            torch.cuda.set_device(self.decoder.first_device)
1416            hidden_states = hidden_states.to(self.decoder.first_device)
                   ...
``` 

https://github.com/huggingface/transformers/blob/main/src/transformers/models/t5/modeling_t5.py#L1411
<|||||>In T5ForConditionalGeneration,

```
1619        if self.model_parallel:
1620            torch.cuda.set_device(self.decoder.first_device)

1622        if labels is not None and decoder_input_ids is None and decoder_inputs_embeds is None:
1623            # get decoder inputs from shifting lm labels to the right
1624           decoder_input_ids = self._shift_right(labels)

1626        # Set device for model parallelism
1627        if self.model_parallel:
1628           torch.cuda.set_device(self.decoder.first_device)
1629           hidden_states = hidden_states.to(self.decoder.first_device)
```

1619-1620 and 1627-1628 are same code. But If 1622-1624 need `set_device`, 1619-1620 is necessary.

https://github.com/huggingface/transformers/blob/main/src/transformers/models/t5/modeling_t5.py#L1619