Fix `MobileViTV2` checkpoint name
# What does this PR do?

For `tests/models/mobilevitv2/test_modeling_mobilevitv2.py::MobileViTV2ModelTest::test_model_from_pretrained`,  we get
```bash
(line 433)  OSError: apple/mobilevitv2-1.0-imagenet1k-256 is not a local folder and is not a valid model identifier listed on 'https://huggingface.co/models'
```

This PR changes the checkpoint name to avoid this failure.

**Question**: Should we upload the model to `apple/mobilevitv2-1.0-imagenet1k-256` and change all checkpoint names in the code to `apple/mobilevitv2-1.0-imagenet1k-256`?

cc @shehanmunasinghe 

_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh Sorry, my bad, I'd assumed because the checkpoints in the PR were under the org they were already uploaded and didn't verify. 

> Question: Should we upload the model to apple/mobilevitv2-1.0-imagenet1k-256 and change all checkpoint names in the code to apple/mobilevitv2-1.0-imagenet1k-256?

Yep! <|||||>OK, so I should change everything to `apple` in this PR instead. I am not a member of `apple` on `Hub`. Maybe I can ask @hollance to help me on this?<|||||>Also, I'd like to suggest moving the following checkpoints under `apple` org as well.

```
shehan97/mobilevitv2-1.0-voc-deeplabv3

shehan97/mobilevitv2-2.0-imagenet1k-256

shehan97/mobilevitv2-1.5-voc-deeplabv3
```
<|||||>@amyeroberts checkpoint is uploaded to `apple`. All tests pass now 🙏 <|||||>Thanks. I missed the other 3 ones 😭 <|||||>> Also, I'd like to suggest moving the following checkpoints under `apple` org as well.
> 
> ```
> shehan97/mobilevitv2-1.0-voc-deeplabv3
> 
> shehan97/mobilevitv2-2.0-imagenet1k-256
> 
> shehan97/mobilevitv2-1.5-voc-deeplabv3
> ```

Thank you @shehanmunasinghe for the heads up 🤗 