Fix MobileVitV2 doctest checkpoint
# What does this PR do?

Doctests CI currently fails on MobileVitV2 tests. Doctest was copied from the V1 mobilevit, and checkpoints didn't match. Remove the copied from as there's just some very simple and standard model head logic coped. 

Interestingly, the [example for MobileVit V1](https://github.com/huggingface/transformers/blob/21946a8cf4a273f35ac2f3a53edafc398699f527/src/transformers/models/mobilevit/modeling_mobilevit.py#L1027) I'd expect to fail as it doesn't import torch. [It is included in the doc tests](https://github.com/huggingface/transformers/blob/21946a8cf4a273f35ac2f3a53edafc398699f527/utils/documentation_tests.txt#L300).

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] 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?
_The documentation is not available anymore as the PR was closed or merged._<|||||>> So in MobileVitV2 doc example, the torch has to be imported?

No, not to have the test pass. I added it because it wouldn't run if you copy-pasted the snippet from the docs.

I've added it to MobileVit v1 too now for the same reason :)