generator` yielded an element that could not be converted to the expected type. The expected type was int32, but the yielded element was None.
# 🐛 Bug

## Information

Model I am using (RoBerta):

Language I am using the model on (English):

The problem arises when using:
* [ ] the official example scripts: (give details below)
inputs = tokenizer(
                text_a,
                text_b,
                add_special_tokens=True,
                max_length=max_length,
                padding="max_length",
                truncation=True,
                return_overflowing_tokens=True,
            )
            if "num_truncated_tokens" in inputs and inputs["num_truncated_tokens"] > 0:
                logger.info(
                    "Attention! you are cropping tokens (swag task is ok). "
                    "If you are training ARC and RACE and you are poping question + options,"
                    "you need to try to use a bigger max seq length!"
                )

            choices_inputs.append(inputs)

        label = label_map[example.label]

        input_ids = [x["input_ids"] for x in choices_inputs]
        attention_mask = (
            [x["attention_mask"] for x in choices_inputs] if "attention_mask" in choices_inputs[0] else None
        )
        token_type_ids = (
            [x["token_type_ids"] for x in choices_inputs] if "token_type_ids" in choices_inputs[0] else None
        )

"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

self.dataset = tf.data.Dataset.from_generator(
                gen,
                (
                    {
                        "example_id": tf.int32,
                        "input_ids": tf.int32,
                        "attention_mask": tf.int32,
                        "token_type_ids": tf.int32,
                    },
                    tf.int64,
                ),
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
The inputs  dose not include the key"token_type_ids",so a None is return,but a None can't convert to tf.int32 in the tensorflow2,so the code can't work

## To reproduce

Steps to reproduce the behavior:

1.
2.
3.

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version:
- Platform:
- Python version:
- PyTorch version (GPU?):
- Tensorflow version (GPU?):
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

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