Add missing autocast() in Trainer.prediction_step()
Author: juice500ml
Pull request: yes

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On prediction_step() where `has_labels == True` and `self.use_amp == True`, it does not use `autocast()` in calling `self.compute_loss`, which triggers `RuntimeError: Input type (torch.cuda.FloatTensor) and weight type (torch.cuda.HalfTensor) should be the same`. This commit fixes the issue.


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