Allow trainer to return eval. loss for CLIP-like models
# What does this PR do?

Allow trainer to give **evaluation** loss for CLIP-like models.

Currently, this line
https://github.com/huggingface/transformers/blob/07d8d6e2f7a920d399e5e86a82d78179cdfa6746/src/transformers/trainer.py#L3192
gives `has_labels = False` for CLIP-like models, and can't give loss value in the evaluation.

without this PR:
```bash
***** eval metrics *****
  epoch                   =        1.0
  eval_runtime            = 0:00:01.67
  eval_samples_per_second =      9.571
  eval_steps_per_second   =      4.785

```

with this PR.
```bash
***** eval metrics *****
  epoch                   =        1.0
  eval_loss               =     0.8159
  eval_runtime            = 0:00:01.66
  eval_samples_per_second =      9.598
  eval_steps_per_second   =      4.799

```
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger Hopefully the change covers everything that could happen now and in the future.