add conditional statement for auxiliary loss calculation
# What does this PR do?

Missing conditional statement before computing auxiliary loss for BeitForSemanticSegmentation. 
File: https://github.com/huggingface/transformers/blob/main/src/transformers/models/beit/modeling_beit.py
Function: BeitForSemanticSegmentation -> compute_loss (Line no.: 1183 - 1198)

```python
    def compute_loss(self, logits, auxiliary_logits, labels):
        # upsample logits to the images' original size
        upsampled_logits = nn.functional.interpolate(
            logits, size=labels.shape[-2:], mode="bilinear", align_corners=False
        )
        if auxiliary_logits is not None:
            upsampled_auxiliary_logits = nn.functional.interpolate(
                auxiliary_logits, size=labels.shape[-2:], mode="bilinear", align_corners=False
            )
        # compute weighted loss
        loss_fct = CrossEntropyLoss(ignore_index=self.config.semantic_loss_ignore_index)
        main_loss = loss_fct(upsampled_logits, labels)
        auxiliary_loss = loss_fct(upsampled_auxiliary_logits, labels)
        loss = main_loss + self.config.auxiliary_loss_weight * auxiliary_loss

        return loss
```

When training BeitForSemanticSegmentation with config["use_auxiliary_head"] set to False, the following error occurs,

```bash
  File "/home/jovyan/segmentation/train.py", line 311, in <module>
    trainer.train()
  File "/usr/local/lib/python3.10/dist-packages/transformers/trainer.py", line 1664, in train
    return inner_training_loop(
  File "/usr/local/lib/python3.10/dist-packages/transformers/trainer.py", line 1940, in _inner_training_loop
    tr_loss_step = self.training_step(model, inputs)
  File "/usr/local/lib/python3.10/dist-packages/transformers/trainer.py", line 2735, in training_step
    loss = self.compute_loss(model, inputs)
  File "/usr/local/lib/python3.10/dist-packages/transformers/trainer.py", line 2767, in compute_loss
    outputs = model(**inputs)
  File "/usr/local/lib/python3.10/dist-packages/torch/nn/modules/module.py", line 1501, in _call_impl
    return forward_call(*args, **kwargs)
  File "/usr/local/lib/python3.10/dist-packages/transformers/models/beit/modeling_beit.py", line 1276, in forward
    loss = self.compute_loss(logits, auxiliary_logits, labels)
  File "/usr/local/lib/python3.10/dist-packages/transformers/models/beit/modeling_beit.py", line 1195, in compute_loss
    auxiliary_loss = loss_fct(upsampled_auxiliary_logits, labels)
UnboundLocalError: local variable 'upsampled_auxiliary_logits' referenced before assignment
```

Possible solution: add a conditional statement before computing the auxiliary loss.

```python
    def compute_loss(self, logits, auxiliary_logits, labels):
        # upsample logits to the images' original size
        upsampled_logits = nn.functional.interpolate(
            logits, size=labels.shape[-2:], mode="bilinear", align_corners=False
        )
        if auxiliary_logits is not None:
            upsampled_auxiliary_logits = nn.functional.interpolate(
                auxiliary_logits, size=labels.shape[-2:], mode="bilinear", align_corners=False
            )
        # compute weighted loss
        loss_fct = CrossEntropyLoss(ignore_index=self.config.semantic_loss_ignore_index)
        main_loss = loss_fct(upsampled_logits, labels)
        loss = main_loss
        if auxiliary_logits is not None:
            auxiliary_loss = loss_fct(upsampled_auxiliary_logits, labels)
            loss += self.config.auxiliary_loss_weight * auxiliary_loss

        return loss
```

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