assertion failed: [predictions must be >= 0]
Trying to train a binary classifier over sentence pairs with custom dataset throws a Tensroflow error.

## Environment info
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     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: `4.2.2`
- Platform: Ubuntu 18.04
- Python version: `3.7.5`
- PyTorch version (GPU?):
- Tensorflow version (GPU): `2.3.1`
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: Nope

### Who can help
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Models:

- albert, bert, xlm: @LysandreJik
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Library:

- benchmarks: @patrickvonplaten
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- tokenizers: @n1t0, @LysandreJik
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- pipelines: @LysandreJik

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- nlp datasets: [different repo](https://github.com/huggingface/nlp)
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- maintained examples (not research project or legacy): @sgugger, @patil-suraj
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 -->

## Information

Model I am using (TFRoberta, TFXLMRoberta...):

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: https://huggingface.co/transformers/training.html#fine-tuning-in-native-tensorflow-2
* [x] my own task or dataset: 

## To reproduce

Steps to reproduce the behavior:
```python
from transformers import TFAutoModelForSequenceClassification, AutoTokenizer
from keras.metrics import Precision, Recall
import tensorflow as tf

def build_dataset(tokenizer, filename):
    data = [[], [], []]
    with open(filename, 'r') as file_:
        for line in file_:
            fields = line.split('\t')
            data[0].append(fields[0].strip())
            data[1].append(fields[1].strip())
            data[2].append(int(fields[2].strip()))
    sentences = tokenizer(data[0], data[1],
                          padding=True,
                          truncation=True)

    return tf.data.Dataset.from_tensor_slices((dict(sentences),
                                             data[2]))
settings = {
    "model": 'roberta-base',
    "batch_size": 8,
    "n_classes": 1,
    "epochs": 10,
    "steps_per_epoch": 128,
    "patience": 5,
    "loss": "binary_crossentropy",
    "lr": 5e7,
    "clipnorm": 1.0,
}
tokenizer = AutoTokenizer.from_pretrained(settings["model"])

train_dataset = build_dataset(tokenizer, 'train.head')
train_dataset = train_dataset.shuffle(
        len(train_dataset)).batch(settings["batch_size"])

dev_dataset = build_dataset(tokenizer, 'dev.head').batch(
        settings["batch_size"])

model = TFAutoModelForSequenceClassification.from_pretrained(
            settings['model'],
            num_labels=1)
model.compile(optimizer='adam',
              #loss='binary_crossentropy',
              loss=model.compute_loss,
              metrics=[Precision(name='p'), Recall(name='r')])
model.summary()
model.fit(train_dataset,
          epochs=settings["epochs"],
          #steps_per_epoch=steps_per_epoch,
          validation_data=dev_dataset,
          batch_size=settings["batch_size"],
          verbose=1)
```

Gives the following output
```
Some layers of TFRobertaForSequenceClassification were not initialized from the model checkpoint at roberta-base and are newly initialized: ['classifier']
You should probably TRAIN this model on a down-stream task to be able to use it for predictions and inference.
Model: "tf_roberta_for_sequence_classification"
_________________________________________________________________
Layer (type)                 Output Shape              Param #
=================================================================
roberta (TFRobertaMainLayer) multiple                  124055040
_________________________________________________________________
classifier (TFRobertaClassif multiple                  591361
=================================================================
Total params: 124,646,401
Trainable params: 124,646,401
Non-trainable params: 0
_________________________________________________________________
Epoch 1/10
The parameters `output_attentions`, `output_hidden_states` and `use_cache` cannot be updated when calling a model.They have to be set to True/False in the config object (i.e.: `config=XConfig.from_pretrained('name', output_attentions=True)`).
The parameter `return_dict` cannot be set in graph mode and will always be set to `True`.
The parameters `output_attentions`, `output_hidden_states` and `use_cache` cannot be updated when calling a model.They have to be set to True/False in the config object (i.e.: `config=XConfig.from_pretrained('name', output_attentions=True)`).
The parameter `return_dict` cannot be set in graph mode and will always be set to `True`.
Traceback (most recent call last):
  File "finetune.py", line 52, in <module>
    verbose=1)
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/keras/engine/training.py", line 108, in _method_wrapper
    return method(self, *args, **kwargs)
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/keras/engine/training.py", line 1098, in fit
    tmp_logs = train_function(iterator)
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/eager/def_function.py", line 780, in __call__
    result = self._call(*args, **kwds)
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/eager/def_function.py", line 840, in _call
    return self._stateless_fn(*args, **kwds)
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/eager/function.py", line 2829, in __call__
    return graph_function._filtered_call(args, kwargs)  # pylint: disable=protected-access
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/eager/function.py", line 1848, in _filtered_call
    cancellation_manager=cancellation_manager)
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/eager/function.py", line 1924, in _call_flat
    ctx, args, cancellation_manager=cancellation_manager))
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/eager/function.py", line 550, in call
    ctx=ctx)
  File "/work/user/bicleaner-neural/venv/lib/python3.7/site-packages/tensorflow/python/eager/execute.py", line 60, in quick_execute
    inputs, attrs, num_outputs)
tensorflow.python.framework.errors_impl.InvalidArgumentError: 2 root error(s) found.
  (0) Invalid argument:  assertion failed: [predictions must be >= 0] [Condition x >= y did not hold element-wise:] [x (tf_roberta_for_sequence_classification/classifier/out_proj/BiasAdd:0) = ] [[0.153356239][0.171548933][0.121127911]...] [y (Cast_3/x:0) = ] [0]
         [[{{node assert_greater_equal/Assert/AssertGuard/else/_1/assert_greater_equal/Assert/AssertGuard/Assert}}]]
         [[assert_greater_equal_1/Assert/AssertGuard/pivot_f/_31/_205]]
  (1) Invalid argument:  assertion failed: [predictions must be >= 0] [Condition x >= y did not hold element-wise:] [x (tf_roberta_for_sequence_classification/classifier/out_proj/BiasAdd:0) = ] [[0.153356239][0.171548933][0.121127911]...] [y (Cast_3/x:0) = ] [0]
         [[{{node assert_greater_equal/Assert/AssertGuard/else/_1/assert_greater_equal/Assert/AssertGuard/Assert}}]]
0 successful operations.
0 derived errors ignored. [Op:__inference_train_function_20780]

Function call stack:
train_function -> train_function
```
The dataset examples look like this:
```python
print(list(train_dataset.take(1).as_numpy_iterator()))
```
```
[({'input_ids': array([[    0,   133,   864, ...,     1,     1,     1],
       [    0,   133,   382, ...,     1,     1,     1],
       [    0,  1121,   645, ...,     1,     1,     1],
       ...,
       [    0,   133,   864, ...,     1,     1,     1],
       [    0,  1121,   144, ...,     1,     1,     1],
       [    0,   495, 21046, ...,     1,     1,     1]], dtype=int32), 'attention_mask': array([[1, 1, 1, ..., 0, 0, 0],
       [1, 1, 1, ..., 0, 0, 0],
       [1, 1, 1, ..., 0, 0, 0],
       ...,
       [1, 1, 1, ..., 0, 0, 0],
       [1, 1, 1, ..., 0, 0, 0],
       [1, 1, 1, ..., 0, 0, 0]], dtype=int32)}, array([0, 0, 0, 0, 1, 0, 0, 0], dtype=int32))]
```

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## Expected behavior

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

Maybe @jplu has an idea!<|||||>I dug up a bit into the model and it seems that the activation is `tanh`. Shouldn't it be a `sigmoid` or a `softmax`? That's why it is producing predictions lower than 0.

```python
In [2]: model.classifier.dense
Out[2]: <tensorflow.python.keras.layers.core.Dense at 0x7f6f70357350>

In [3]: model.classifier.dense.activation
Out[3]: <function tensorflow.python.keras.activations.tanh(x)>
```<|||||>Hello!

From what I can see from your dataset example, you have two labels `0` and `1` and not one so that's might be why you get this issue. For regression task, (single label output), they all have to have a float value between `0` and `1`. You can have an example with the `stsb` glue task in our example here https://github.com/huggingface/transformers/blob/master/examples/text-classification/run_tf_glue.py<|||||>Alright, I'll take a look then. Predicting a binary classification with 2 neurons and `linear` or `tanh` activations seemed strange to me. I've always used a single neuron with `sigmoid`.<|||||>For binary classification, you have two labels and then neurons, it is more intuitive to proceed that way :) but yes you can also do what you propose and round the float value to 0 or 1 depending of the output of your sigmoid activation. Nevertheless, our models don't propose such approach.<|||||>Figured out that the error was thrown by the `Precision` and `Recall` classes because they require values between 0 and 1. In case someone wants to use them when training with native TensowFlow I managed to add the `argmax` to the the classes with this:

```python
from tensorflow.python.keras.utils import metrics_utils

class PrecisionArgmax(Precision):
    def update_state(self, y_true, y_pred, sample_weight=None):
        y_pred = tf.math.argmax(y_pred, -1)
        return metrics_utils.update_confusion_matrix_variables(
            {
                metrics_utils.ConfusionMatrix.TRUE_POSITIVES: self.true_positives,
                metrics_utils.ConfusionMatrix.FALSE_POSITIVES: self.false_positives
            },
            y_true,
            y_pred,
            thresholds=self.thresholds,
            top_k=self.top_k,
            class_id=self.class_id,
            sample_weight=sample_weight)
```

So the code that I posted works with `num_classes=2` and using the the overridden classes as metrics.