Fixed FLAVA tensor masking
Author: amariucaitheodor
Pull request: yes

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Fixes #23378


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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: HF Trainer: @stas00, Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger, @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23384). All of your documentation changes will be reflected on that endpoint.

Hi @amariucaitheodor, thanks for reporting the issue and for opening this PR to resolve it! 🚀 

Going through the code, I think we can simplify the original logic a bit, which would remove the need for these additional checks: we can simply remove the `sequence_for_text = sequence_for_text[pos_mask]` and `sequence_for_image = sequence_for_image[pos_mask]` blocks. 

As either: 
* `pos_mask` is not `None` - then `multimodal_masked_embeddings` will have been masked and `sequence_for_image = multimodal_masked_embeddings` or
* `pos_mask` is `None` - then `multimodal_masked_embeddings` won't have been masked and `sequence_for_image = multimodal_masked_embeddings`

I noticed two additional related pieces which would be great to add to this PR too: 
* `bool_masked_pos` isn't masked in the `ITM Loss` loss block, and should be after `mim_labels`
* We don't need the `if multimodal_masked_embeddings is not None:` check on L1949 - `multimodal_masked_embeddings` is never `None` in this ITM loss block. 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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Hi @amariucaitheodor, are you still working on this PR? We'll want to add these changes, and merging in this branch means you'll get the contribution :) 

Hello @amyeroberts, thank you for the additions and reminder! I can push my changes to GitHub around the 22nd of June. 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.