Better messaging and fix for incorrect shape when collating data.
# What does this PR do?

I ran into an error related to an incorrect shape of inputs when using DataCollatorForSeq2Seq. I learned it had to do with having excessively nested inputs  for my features. The error message was not particularly useful.

This PR adds an assertion checking for incorrectly shaped inputs to be collated. The assertion also provides a solution by suggesting to use `remove_excess_nesting` util.
`remove_excess_nesting` removes excessive nesting from features within a `DatasetDict`.

Fixes #15505


## 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/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?
@stas00 

_The documentation is not available anymore as the PR was closed or merged._