Only count num items in batch when needed
Author: IlyasMoutawwakil
Pull request: yes

# What does this PR do?

Similar to https://github.com/huggingface/transformers/pull/36861 this PR reduces the overhead from calculating `num_items_in_batch` when it's not needed, e.g in cases like image classification where models don't take loss kwargs.

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Comments:
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@IlyasMoutawwakil  Could we consider incorporating this variable into the Trainer constructor, for example at line https://github.com/huggingface/transformers/blob/6a2627918d84f25422b931507a8fb9146106ca20/src/transformers/trainer.py#L643

```py
        self.neftune_noise_alpha = args.neftune_noise_alpha

        self.count_num_items_in_batch = (
                # num_items_in_batch is passed to model forward
                # https://github.com/huggingface/transformers/blob/v4.49.0/src/transformers/trainer.py#L3757
                self.model_accepts_loss_kwargs
                # num_items_in_batch is passed to compute_loss_func
                # https://github.com/huggingface/transformers/blob/v4.49.0/src/transformers/trainer.py#L3773
                or self.compute_loss_func is not None
                # num_items_in_batch is also verified if (self.model_accepts_loss_kwargs or self.compute_loss_func)
                # https://github.com/huggingface/transformers/blob/v4.49.0/src/transformers/trainer.py#L3790
            )
```

and later just use it 

```py
        count_num_items_in_batch = (
            self.count_num_items_in_batch
            and len(batch_samples) > 0
            and "labels" in batch_samples[0]
        )
```

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_36867). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@yafshar it's only used in one place so I don't see a reason for that.