BeitForMaskedImageModeling forward not using bool_masked_pos


### Who can help
@NielsRogge

## Information

I am reading the code of BEIT, trying to use the `BeitForMaskedImageModeling`, while I found the `bool_masked_pos` is defined but not used in `BeitForMaskedImageModeling`'s forward.

In my understanding, `bool_masked_pos` is used to mask out the input image tokens, thus should be passed to the `self.beit` forward here.
https://github.com/huggingface/transformers/blob/5c6eca71a983bae2589eed01e5c04fcf88ba5690/src/transformers/models/beit/modeling_beit.py#L723

Furthermore, the documentation of the `BeitForMaskedImageModeling`'s `forward` lack the description of the `bool_masked_pos`.



Hi,

Thanks for looking into this model. Looking at it now, it's indeed a mistake, not sure why I forgot to pass `bool_masked_pos` to `BeitModel`.

I have a Colab notebook, in which I tried to reconstruct DALL-E's visual tokens using `BeitForMaskedImageModeling`: https://colab.research.google.com/drive/1Mjt-3jHw9HYMXECmSdDlbiG59ZAw-Z0T?usp=sharing#scrollTo=ZwTO9fbhPOxi

It was not working as expected, so probably that'll be the mistake.

> Furthermore, the documentation of the BeitForMaskedImageModeling's forward lack the description of the bool_masked_pos.

This is a good point and should be added. 

I will fix both in a PR.

Thanks!