Repository: transformers
Issue: Export LayoutLMv2 to onnx 
Labels: Good First Issue

I am trying to export LayoutLMv2 model to onnx but there is no support for that available in transformers library.
I have tried to follow the method available for layoutLM but that is not working. 
Here is config class for LayoutLMv2
```
class LayoutLMv2OnnxConfig(OnnxConfig):
    def __init__(
        self,
        config: PretrainedConfig,
        task: str = "default",
        patching_specs: List[PatchingSpec] = None,
    ):
        super().__init__(config, task=task, patching_specs=patching_specs)
        self.max_2d_positions = config.max_2d_position_embeddings - 1

    @property
    def inputs(self) -> Mapping[str, Mapping[int, str]]:
        return OrderedDict(
            [
                ("input_ids", {0: "batch", 1: "sequence"}),
                ("bbox", {0: "batch", 1: "sequence"}),
                ("image", {0: "batch", 1: "sequence"}),
                ("attention_mask", {0: "batch", 1: "sequence"}),
                ("token_type_ids", {0: "batch", 1: "sequence"}),
            ]
        )

    def generate_dummy_inputs(
        self,
        tokenizer: PreTrainedTokenizer,
        batch_size: int = -1,
        seq_length: int = -1,
        is_pair: bool = False,
        framework: Optional[TensorType] = None,
    ) -> Mapping[str, Any]:
        """
        Generate inputs to provide to the ONNX exporter for the specific framework
        Args:
            tokenizer: The tokenizer associated with this model configuration
            batch_size: The batch size (int) to export the model for (-1 means dynamic axis)
            seq_length: The sequence length (int) to export the model for (-1 means dynamic axis)
            is_pair: Indicate if the input is a pair (sentence 1, sentence 2)
            framework: The framework (optional) the tokenizer will generate tensor for
        Returns:
            Mapping[str, Tensor] holding the kwargs to provide to the model's forward function
        """

        input_dict = super().generate_dummy_inputs(tokenizer, batch_size, seq_length, is_pair, framework)

        # Generate a dummy bbox
        box = [48, 84, 73, 128]

        if not framework == TensorType.PYTORCH:
            raise NotImplementedError("Exporting LayoutLM to ONNX is currently only supported for PyTorch.")

        if not is_torch_available():
            raise ValueError("Cannot generate dummy inputs without PyTorch installed.")
        import torch

        batch_size, seq_length = input_dict["input_ids"].shape
        input_dict["bbox"] = torch.tensor([*[box] * seq_length]).tile(batch_size, 1, 1)
        return input_dict

onnx_config = LayoutLMv2OnnxConfig(model.config)


export(tokenizer=tokenizer, model=model, config=onnx_config, opset=12, output=Path('onnx/layoutlmv2.onnx'))
```

Running the export line is raising this error,

```
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-25-99a1f167e396> in <module>()
----> 1 export(tokenizer=tokenizer, model=model, config=onnx_config, opset=12, output=Path('onnx/layoutlmv2.onnx'))

3 frames
/usr/local/lib/python3.7/dist-packages/transformers/models/layoutlmv2/tokenization_layoutlmv2.py in __call__(self, text, text_pair, boxes, word_labels, add_special_tokens, padding, truncation, max_length, stride, pad_to_multiple_of, return_tensors, return_token_type_ids, return_attention_mask, return_overflowing_tokens, return_special_tokens_mask, return_offsets_mapping, return_length, verbose, **kwargs)
    449 
    450         words = text if text_pair is None else text_pair
--> 451         assert boxes is not None, "You must provide corresponding bounding boxes"
    452         if is_batched:
    453             assert len(words) == len(boxes), "You must provide words and boxes for an equal amount of examples"

AssertionError: You must provide corresponding bounding boxes
```