Replace asserts with exceptions
Hi! I replaced asserts in the following files related to #12789 issue:

- src/transformers/trainer_pt_utils.py
- tests/test_feature_extraction_detr.py
- tests/test_feature_extraction_clip.py
- tests/test_feature_extraction_common.py
- tests/test_trainer_seq2seq.py

## Before submitting
* [ ]  This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
* [x]  Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?
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* [ ]  Did you make sure to update the documentation with your changes? Here are the [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
* [ ]  Did you write any new necessary tests?

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@sgugger Please review