Fix type annotations for `distributed_concat()`
# What does this PR do?

Fix the type annotations for [`trainer_pt_utils.distributed_concat()`](https://github.com/huggingface/transformers/blob/91df45516c9c21283df21d564bc352067d8c5f62/src/transformers/trainer_pt_utils.py#L160).

The input of this function could be a `torch.Tensor` or `list` / `tuple`, so I guess the type annotations:

https://github.com/huggingface/transformers/blob/91df45516c9c21283df21d564bc352067d8c5f62/src/transformers/trainer_pt_utils.py#L160

should be:

```python
def distributed_concat(
    tensor: Union[torch.Tensor, Any], num_total_examples: Optional[int] = None
) -> Union[torch.Tensor, Any]:
```


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- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?


## Who can review?

@sgugger
Done.