Fix typing error in Trainer class (prediction_step)
Author: jannisborn
Pull request: yes

# What does this PR do?

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This is a minor fix. The argument types and docstring of `transformers.trainer.prediction_step` are incorrect. This error was introduced in transformers==3.4.0, specifically in #7767 where documentation was not updated properly.

The current docs indicate that `prediction_step` returns a 3-Tuple of Optionals (loss, logits and labels) and that the type of _loss_ is `float`. Indeed, if returned, `loss` is always a `torch.Tensor` as the only performed operations in this function are `.mean()`, `.detach()` and `.cpu()`, but **not** `.item()`.  In transformers<3.4.0, there was indeed a `.item()` operation, but in #7767 this behavior was changed but the docstring and types were not updated.



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