Remove masked image modeling from BEIT ONNX export
# What does this PR do?

This PR removes masked image modeling from the list of supported features in the ONNX exporter. As explained by @NielsRogge, BEiT cannot be loaded with the `AutoModelForMaskedImageModeling` class due to:

> Well yeah that's because BEiT does masked image modeling by predicting visual tokens of a VQ-VAE, whereas the other ones predict pixel values (RGB) as in the [SimMIM paper](https://arxiv.org/abs/2111.09886). So I'm afraid BEiT cannot be added to this auto class.

I've also added a note in the BEiT docs to help users who don't know these details. I've also checked that the slow tests pass for ONNX with

```
RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -s
```

Edit: we should merge this after #16981 to ensure the RoFormer tests pass first
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi,

There's a reason I haven't added BEiT to the auto classes. It's because it can't be used with the run_mim.py script, because BEiT handles masked image modeling differently compared to the other ones (which do it similar to the way it's defined in SimMIM paper). So this may confuse users, maybe we should properly document it that BEiT is not the same as the other ones<|||||>Ah I see, but isn't a bit odd to exclude BEiT just because it isn't compatible with our example scripts? 

For instance, is there anything fundamentally wrong with loading `BeitForMaskedImageModeling` via the autoclass if I'm rolling my own masked image modeling code?

If not, I'd prefer to keep BEIT in the autoclasses and put the warning inside the `run_mim.py` script if a user tries to run it with this architecture<|||||>Hmm maybe there is a fundamental issue with using BEiT in the autoclasses as I'm seeing the torch tests fail with:

```
self = BeitEmbeddings(
  (patch_embeddings): PatchEmbeddings(
    (projection): Conv2d(3, 32, kernel_size=(2, 2), stride=(2, 2))
  )
  (dropout): Dropout(p=0.1, inplace=False)
)
pixel_values = tensor([[[[7.7614e-01, 1.7656e-01, 6.0460e-01,  ..., 3.9106e-01,
           5.2019e-01, 8.9339e-01],
          [2.7568...1, 9.9367e-01],
          [9.4963e-01, 1.6943e-01, 9.7946e-01,  ..., 1.9085e-01,
           1.9910e-01, 4.6059e-02]]]])
bool_masked_pos = tensor([[0, 0, 0,  ..., 0, 0, 0],
        [0, 0, 0,  ..., 0, 0, 0],
        [0, 0, 0,  ..., 0, 0, 0],
        ...,
        [0, 0, 0,  ..., 0, 0, 0],
        [0, 0, 0,  ..., 0, 0, 0],
        [0, 0, 0,  ..., 0, 0, 0]])

    def forward(self, pixel_values: torch.Tensor, bool_masked_pos: Optional[torch.BoolTensor] = None) -> torch.Tensor:
    
        embeddings = self.patch_embeddings(pixel_values)
        batch_size, seq_len, _ = embeddings.size()
    
        cls_tokens = self.cls_token.expand(batch_size, -1, -1)
        if bool_masked_pos is not None:
>           mask_tokens = self.mask_token.expand(batch_size, seq_len, -1)
E           AttributeError: 'NoneType' object has no attribute 'expand'
```<|||||>Well yeah that's because BEiT does masked image modeling by predicting visual tokens of a VQ-VAE, whereas the other ones predict pixel values (RGB) as in the [SimMIM paper](https://arxiv.org/abs/2111.09886). So I'm afraid BEiT cannot be added to this auto class.<|||||>OK thanks for the clarification. I'll remove this feature from the ONNX export and add a note to the BEiT docs :)