tf trainer dataset cardinality issue - potentially a bug
In `trainer_tf.py`, line 138, we have

    self.num_train_examples = self.train_dataset.cardinality(self.train_dataset).numpy()

in the method 

    def get_train_tfdataset(self) -> tf.data.Dataset:

However, in the official tf documenation [cardinality](https://www.tensorflow.org/api_docs/python/tf/data/Dataset#cardinality), it is defined as:

    cardinality()

which has no argument.

I got the following error

```
    File "/home/imo/Desktop/transformers/src/transformers/trainer_tf.py", line 138, in get_train_tfdataset
      self.num_train_examples = self.train_dataset.cardinality(self.train_dataset).numpy()
    TypeError: cardinality() takes 1 positional argument but 2 were given
```
    
I think the current version on master is a bug, which should be changed to 

    self.train_dataset.cardinality().numpy()


Could you confirm, @jplu? And if it is a bug, let's fix it. Thank you.
I can confirm! Good catch!<|||||>I resume the work on creating `test_trainer_tf.py`, I promise I will finish it this time. After that, it might be easier to catch the errors in `tf_trainer.py`.<|||||>I take care of this!<|||||>OK, @jplu . Thank you for letting me know about it (I did some check and didn't found it on master, so I thought it was not done yet).