fix trainer tr_loss add error
Author: Wang-Xiaodong1899
Pull request: yes

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Fixes tr_loss += xxx error. The error is RuntimeError: output with shape [] doesn't match the broadcast shape [1].

As `tr_loss` is defined as `tr_loss = torch.tensor(0.0).to(args.device)` in https://github.com/huggingface/transformers/blob/main/src/transformers/trainer.py#L2274

An example:
![{3223466F-FA20-4236-9DFD-A7FDF0650FF5}](https://github.com/user-attachments/assets/2c0d0d61-aa04-4c0e-a805-c6bbd83b0766)

So, a safe way it to use tr_loss = tr_loss + xxx, avoiding the error.


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