Tensorflow Glue example script for finetuning not usable with DistilBert
# 🐛 Bug

## Information

Model I am using: DistilBert

```
python run_glue.py \
  --model_name_or_path distilbert-base-cased \
  --task_name MRPC\
  --do_train \
  --do_eval \
  --max_seq_length 128 \
  --per_device_train_batch_size 32 \
  --learning_rate 2e-5 \
  --num_train_epochs 3.0 \
  --output_dir /tmp/distilbert/
```

Language I am using the model on (English, Chinese ...): English

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

Running the example as above, the script gives me the following error:
```
Traceback (most recent call last):
  File "run_glue.py", line 229, in <module>
    main()
  File "run_glue.py", line 199, in main
    compute_metrics=compute_metrics,
  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/transformers/trainer_tf.py", line 48, in __init__
    self._setup_training()
  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/transformers/trainer_tf.py", line 58, in _setup_training
    self._prepare_dataset()
  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/transformers/trainer_tf.py", line 95, in _prepare_dataset
    self.num_train_examples = self.train_dataset.reduce(tf.constant(0), lambda x, _: x + 1).numpy()
  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/data/ops/dataset_ops.py", line 1934, in reduce
    output_types=structure.get_flat_tensor_types(state_structure)))
  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/ops/gen_dataset_ops.py", line 4661, in reduce_dataset
    _ops.raise_from_not_ok_status(e, name)
  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/framework/ops.py", line 6606, in raise_from_not_ok_status
    six.raise_from(core._status_to_exception(e.code, message), None)
  File "<string>", line 3, in raise_from
tensorflow.python.framework.errors_impl.InvalidArgumentError: TypeError: `generator` yielded an element that could not be converted to the expected type. The expected type was int32, but the yielded element was None.
Traceback (most recent call last):

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/data/ops/dataset_ops.py", line 805, in generator_py_func
    ret, dtype=dtype.as_numpy_dtype))

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/ops/script_ops.py", line 196, in _convert
    result = np.asarray(value, dtype=dtype, order="C")

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/numpy/core/_asarray.py", line 85, in asarray
    return array(a, dtype, copy=False, order=order)

TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'


During handling of the above exception, another exception occurred:


Traceback (most recent call last):

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/ops/script_ops.py", line 236, in __call__
    ret = func(*args)

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/data/ops/dataset_ops.py", line 810, in generator_py_func
    "element was %s." % (dtype.name, ret)), sys.exc_info()[2])

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/six.py", line 702, in reraise
    raise value.with_traceback(tb)

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/data/ops/dataset_ops.py", line 805, in generator_py_func
    ret, dtype=dtype.as_numpy_dtype))

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/tensorflow_core/python/ops/script_ops.py", line 196, in _convert
    result = np.asarray(value, dtype=dtype, order="C")

  File "/home/ubuntu/anaconda3/envs/tensorflow2_p36/lib/python3.6/site-packages/numpy/core/_asarray.py", line 85, in asarray
    return array(a, dtype, copy=False, order=order)

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


	 [[{{node PyFunc}}]] [Op:ReduceDataset]
```

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: MRPC
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. clone repo
2. run command from above using `examples/text-classification/run_tf_glue.py`

## Expected behavior
Fine tuning works on Distilbert too, not only on Bert.

## 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: 2.11.0
- Platform: Linux-5.3.0-1017-aws-x86_64-with-debian-buster-sid
- Python version: 3.6.10
- PyTorch version (GPU?): not installed (NA)
- Tensorflow version (GPU?): 2.1.0 (True)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

Hello!

Indeed, it is a bug in the way the TensorFlow dataset is generated. A fix is on its way :)