MarkupLM: TypeError: unsupported operand type(s) for +: 'Tensor' and 'NoneType' in modeling_markuplm.py", line 217
### System Info

- `transformers` version: 4.29.2
- Platform: Linux-6.1.22-aufs-1-x86_64-with-glibc2.31
- Python version: 3.9.2
- Huggingface_hub version: 0.14.1
- Safetensors version: not installed
- PyTorch version (GPU?): 2.0.1+cu117 (False)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>

### Who can help?

_No response_

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

Run the code above or similar, probably for a larger than 512 or 1024 tokens input:
tokenizer = MarkupLMTokenizerFast.from_pretrained("microsoft/markuplm-large-finetuned-websrc")
model = MarkupLMForQuestionAnswering.from_pretrained("microsoft/markuplm-large-finetuned-websrc")
feature_extractor = MarkupLMFeatureExtractor()
feature_processor = MarkupLMProcessor(feature_extractor, tokenizer)

...

encoding = feature_processor(html_content, questions=question, return_tensors="pt")
with torch.no_grad():
        outputs = model(**encoding)

I tried to extend the model's max tokens size as[ suggested here](https://discuss.huggingface.co/t/fine-tuning-bert-with-sequences-longer-than-512-tokens/12652), but that doesn't seem to help

### Expected behavior

outputs = model(**encoding) should create outputs
We cannot help without a clear reproducer of the issue.<|||||>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.