Add on option to output a checkpoint every x minutes
# 🚀 Feature request

Extending `transformers.trainer_utils.IntervalStrategy` by an option `MINUTES` to be able to save a checkpoint based on training time.

## Motivation

For hyper-parameter searches over large batch sizes (where gradient accumulation is necessary) the time between steps can vary by orders of magnitude between different runs. This could lead to very different number of written checkpoints for different runs. In this case it could be useful to specify to have checkpoints output every x hours rather than every x steps. 

## Contribution

Happy to submit a PR for that.

cc @sgugger <|||||>This sounds like a useful feature, feel free to suggest a PR to add that!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Unstale<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Unstale<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.