

TITLE
OperatorNotAllowedInGraphError in dbmdz/bert-base-italian-cased for Token Classification

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## 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: 3.3.1
- Platform: Linux-5.4.0-1028-aws-x86_64-with-debian-buster-sid
- Python version: 3.7.7
- PyTorch version (GPU?): not installed (NA)
- Tensorflow version (GPU?): 2.2.0 (True)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
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 Summarization: @sshleifer
 TextGeneration: @TevenLeScao 
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@LysandreJik @stefan-it

## Information

Model I am using (Bert, XLNet ...):

Bert (dbmdz/bert-base-italian-cased)

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

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

## To reproduce

Steps to reproduce the behavior:
I am following the tutorial in https://huggingface.co/transformers/custom_datasets.html#token-classification-with-w-nut-emerging-entities in order to fine-tune a custom Named Entity Recognition NN.

1. Load the tokenizer and tokenize the data
```
 tokenizer = BertTokenizerFast.from_pretrained('dbmdz/bert-base-italian-cased')
```
following the steps from [tutorial](https://huggingface.co/transformers/custom_datasets.html#token-classification-with-w-nut-emerging-entities) to obtain `train_dataset` and `val_dataset` .

2. Load the Token Classifier with pretrained weights from the model
 ```
from transformers import TFBertForTokenClassification 
model = TFBertForTokenClassification.from_pretrained('dbmdz/bert-base-italian-cased', num_labels=len(unique_tags))
```
As a side note, this gives the following warning which I understand is to be expected since the loaded model was not fine tuned for Token Classification:
```
Some weights of the model checkpoint at dbmdz/bert-base-italian-cased were not used when initializing TFBertForTokenClassification: ['mlm___cls', 'nsp___cls']
- This IS expected if you are initializing TFBertForTokenClassification from the checkpoint of a model trained on another task or with another architecture (e.g. initializing a BertForSequenceClassification model from a BertForPretraining model).
- This IS NOT expected if you are initializing TFBertForTokenClassification from the checkpoint of a model that you expect to be exactly identical (initializing a BertForSequenceClassification model from a BertForSequenceClassification model).
Some weights of TFBertForTokenClassification were not initialized from the model checkpoint at dbmdz/bert-base-italian-cased and are newly initialized: ['dropout_37', 'classifier']
You should probably TRAIN this model on a down-stream task to be able to use it for predictions and inference.
```
4. Train
```
optimizer = tf.keras.optimizers.Adam(learning_rate=5e-5)
model.compile(optimizer=optimizer, loss=model.compute_loss)
model.fit(train_dataset.shuffle(100).batch(16), epochs=3, batch_size=16)
```
This raises the following error:

```
Epoch 1/3

---------------------------------------------------------------------------
OperatorNotAllowedInGraphError            Traceback (most recent call last)
<ipython-input-23-5eb528629965> in <module>
      1 optimizer = tf.keras.optimizers.Adam(learning_rate=5e-5)
      2 model.compile(optimizer=optimizer, loss=model.compute_loss)
----> 3 model.fit(train_dataset.shuffle(100).batch(16), epochs=3, batch_size=16)

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/keras/engine/training.py in _method_wrapper(self, *args, **kwargs)
     64   def _method_wrapper(self, *args, **kwargs):
     65     if not self._in_multi_worker_mode():  # pylint: disable=protected-access
---> 66       return method(self, *args, **kwargs)
     67 
     68     # Running inside `run_distribute_coordinator` already.

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/keras/engine/training.py in fit(self, x, y, batch_size, epochs, verbose, callbacks, validation_split, validation_data, shuffle, class_weight, sample_weight, initial_epoch, steps_per_epoch, validation_steps, validation_batch_size, validation_freq, max_queue_size, workers, use_multiprocessing)
    851                 batch_size=batch_size):
    852               callbacks.on_train_batch_begin(step)
--> 853               tmp_logs = train_function(iterator)
    854               # Catch OutOfRangeError for Datasets of unknown size.
    855               # This blocks until the batch has finished executing.

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/eager/def_function.py in __call__(self, *args, **kwds)
    578         xla_context.Exit()
    579     else:
--> 580       result = self._call(*args, **kwds)
    581 
    582     if tracing_count == self._get_tracing_count():

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/eager/def_function.py in _call(self, *args, **kwds)
    625       # This is the first call of __call__, so we have to initialize.
    626       initializers = []
--> 627       self._initialize(args, kwds, add_initializers_to=initializers)
    628     finally:
    629       # At this point we know that the initialization is complete (or less

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/eager/def_function.py in _initialize(self, args, kwds, add_initializers_to)
    504     self._concrete_stateful_fn = (
    505         self._stateful_fn._get_concrete_function_internal_garbage_collected(  # pylint: disable=protected-access
--> 506             *args, **kwds))
    507 
    508     def invalid_creator_scope(*unused_args, **unused_kwds):

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/eager/function.py in _get_concrete_function_internal_garbage_collected(self, *args, **kwargs)
   2444       args, kwargs = None, None
   2445     with self._lock:
-> 2446       graph_function, _, _ = self._maybe_define_function(args, kwargs)
   2447     return graph_function
   2448 

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/eager/function.py in _maybe_define_function(self, args, kwargs)
   2775 
   2776       self._function_cache.missed.add(call_context_key)
-> 2777       graph_function = self._create_graph_function(args, kwargs)
   2778       self._function_cache.primary[cache_key] = graph_function
   2779       return graph_function, args, kwargs

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/eager/function.py in _create_graph_function(self, args, kwargs, override_flat_arg_shapes)
   2665             arg_names=arg_names,
   2666             override_flat_arg_shapes=override_flat_arg_shapes,
-> 2667             capture_by_value=self._capture_by_value),
   2668         self._function_attributes,
   2669         # Tell the ConcreteFunction to clean up its graph once it goes out of

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/framework/func_graph.py in func_graph_from_py_func(name, python_func, args, kwargs, signature, func_graph, autograph, autograph_options, add_control_dependencies, arg_names, op_return_value, collections, capture_by_value, override_flat_arg_shapes)
    979         _, original_func = tf_decorator.unwrap(python_func)
    980 
--> 981       func_outputs = python_func(*func_args, **func_kwargs)
    982 
    983       # invariant: `func_outputs` contains only Tensors, CompositeTensors,

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/eager/def_function.py in wrapped_fn(*args, **kwds)
    439         # __wrapped__ allows AutoGraph to swap in a converted function. We give
    440         # the function a weak reference to itself to avoid a reference cycle.
--> 441         return weak_wrapped_fn().__wrapped__(*args, **kwds)
    442     weak_wrapped_fn = weakref.ref(wrapped_fn)
    443 

~/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/framework/func_graph.py in wrapper(*args, **kwargs)
    966           except Exception as e:  # pylint:disable=broad-except
    967             if hasattr(e, "ag_error_metadata"):
--> 968               raise e.ag_error_metadata.to_exception(e)
    969             else:
    970               raise

OperatorNotAllowedInGraphError: in user code:

    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/keras/engine/training.py:571 train_function  *
        outputs = self.distribute_strategy.run(
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/distribute/distribute_lib.py:951 run  **
        return self._extended.call_for_each_replica(fn, args=args, kwargs=kwargs)
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/distribute/distribute_lib.py:2290 call_for_each_replica
        return self._call_for_each_replica(fn, args, kwargs)
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/distribute/distribute_lib.py:2649 _call_for_each_replica
        return fn(*args, **kwargs)
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/keras/engine/training.py:533 train_step  **
        y, y_pred, sample_weight, regularization_losses=self.losses)
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/keras/engine/compile_utils.py:205 __call__
        loss_value = loss_obj(y_t, y_p, sample_weight=sw)
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/keras/losses.py:143 __call__
        losses = self.call(y_true, y_pred)
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/keras/losses.py:246 call
        return self.fn(y_true, y_pred, **self._fn_kwargs)
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/transformers/modeling_tf_utils.py:178 compute_loss
        if tf.math.reduce_any(labels == -1):
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/framework/ops.py:778 __bool__
        self._disallow_bool_casting()
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/framework/ops.py:545 _disallow_bool_casting
        "using a `tf.Tensor` as a Python `bool`")
    /home/ubuntu/anaconda3/envs/doc_backoffice/lib/python3.7/site-packages/tensorflow/python/framework/ops.py:532 _disallow_when_autograph_enabled
        " decorating it directly with @tf.function.".format(task))

    OperatorNotAllowedInGraphError: using a `tf.Tensor` as a Python `bool` is not allowed: AutoGraph did not convert this function. Try decorating it directly with @tf.function.
```

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## Expected behavior

I would expect the model to train without errors, thank you for your kind help!
