model fine-tuning error
Author: marcomameli1992
Pull request: no

### System Info

Hi to all,
I fine-tune a model for my dataset. 

But I need some help with the inference execution. I train the model with a tokenizer without truncation, but I receive the first error in inference. So I tried to retrain the model with truncation activated, as shown in the code. Still, I encountered a new error during the training that only appeared after adding truncation into the tokenizer.

Now, if I try to train the network without truncation on the tokenizer, the training not working, and I need help understanding what happens.

### Who can help?

@ArthurZucker 
@sgugger 

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [X] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

Inference error:
RuntimeError: The size of tensor a (726) must match the size of tensor b (512) at non-singleton dimension 1
The function for classification is:
```
def classification_infer(data, model_path):
    # device
    device = find_device()

    # data preprocessing
    data[['notuseful', 'usefull']] = data['Descrizione'].apply(text_splitting)
    data = data.loc[~data['Classe'].isin([0, 1, 2])]

    # model loading
    model = AutoModelForSequenceClassification.from_pretrained(model_path, num_labels=3)
    tokenizer = AutoTokenizer.from_pretrained(model_path)
    print("Model loaded")

    print("Classifying...")
    classifier = pipeline('text-classification', model=model, tokenizer=tokenizer, device=device)
    tokenizer_kwargs = {'padding': True, 'truncation': True, 'max_length': 512, 'return_tensors': 'pt'}
    classifier_output = classifier(data['usefull'].tolist())
    print("Classification completed")
    data['Classe'] = [int(x['label']) for x in classifier_output]

    return data
```




Training function, with truncation active:
```
def classification_train(data):
    # metric
    metric = evaluate.load('accuracy')

    def compute_metrics(eval_pred):
        logits, labels = eval_pred
        predictions = np.argmax(logits, axis=-1)
        return metric.compute(predictions=predictions, references=labels)

    # Load tokenizer
    tokenizer_kwargs = {'padding': True, 'truncation': True, 'max_length': 512, 'return_tensors': 'pt'}
    tokenizer = AutoTokenizer.from_pretrained("dbmdz/bert-base-italian-cased", tokenizer_kwargs=tokenizer_kwargs)

    # preprocessing function
    def preprocess_function(data):
        return tokenizer(data['text'])

    # device
    device = find_device()

    # data preprocessing
    data[['notuseful', 'usefull']] = data['Descrizione'].apply(text_splitting)

    # dataset creation
    dataset = pd.DataFrame()
    dataset[['text', 'label']] = data.loc[data['Classe'].isin([0, 1, 2]), ['usefull', 'Classe']]
    dataset['label'] = dataset['label'].astype(int)

    # dataset equilibrium
    dataset = dataset.groupby('label').head(100)
    dataset['text'] = dataset['text'].map(lambda x: x.lower())

    # dataset split
    train, test = train_test_split(dataset, test_size=0.2, random_state=42)

    # huggingface dataset
    train = Dataset.from_pandas(train)
    train = train.map(preprocess_function, batched=True)
    test = Dataset.from_pandas(test)
    test = test.map(preprocess_function, batched=True)

    # Load model
    model = AutoModelForSequenceClassification.from_pretrained("dbmdz/bert-base-italian-cased", num_labels=3)
    model.to(device)

    # data collator
    data_collator = DataCollatorWithPadding(tokenizer=tokenizer)

    # training arguments
    training_args = TrainingArguments(
        output_dir='./model_weight',  # output directory
        num_train_epochs=2,  # total number of training epochs
        per_device_train_batch_size=16,  # batch size per device during training
        per_device_eval_batch_size=64,  # batch size for evaluation
        weight_decay=0.01,  # strength of weight decay
        evaluation_strategy="epoch",
        save_strategy="epoch",
        load_best_model_at_end=True,
        metric_for_best_model='eval_accuracy',
        greater_is_better=True,
        report_to="wandb",  # enable logging to W&B
        run_name="bert-base-italian-cased-fit-for-crm",  # name of the W&B run (optional)
        label_names=['0', '1', '2']
    )

    # trainer
    trainer = Trainer(
        model=model,  # the instantiated 🤗 Transformers model to be trained
        args=training_args,  # training arguments, defined above
        train_dataset=train,  # training dataset
        eval_dataset=test,  # evaluation dataset
        data_collator=data_collator,  # data collator
        tokenizer=tokenizer,  # tokenizer
        compute_metrics=compute_metrics,  # the callback that computes metrics of interest
    )

    # train
    trainer.train()
```
Error during the evaluation step with the training function with truncation active:
IndexError: too many indices for array: array is 0-dimensional, but 1 were indexed 

training function withouth truncation:
```
def classification_train(data):
    # metric
    metric = evaluate.load('accuracy')

    def compute_metrics(eval_pred):
        logits, labels = eval_pred
        predictions = np.argmax(logits, axis=-1)
        return metric.compute(predictions=predictions, references=labels)

    # Load tokenizer
    tokenizer = AutoTokenizer.from_pretrained("dbmdz/bert-base-italian-cased")

    # preprocessing function
    def preprocess_function(data):
        return tokenizer(data['text'], truncation=False)

    # device
    device = find_device()

    # data preprocessing
    data[['notuseful', 'usefull']] = data['Descrizione'].apply(text_splitting)

    # dataset creation
    dataset = pd.DataFrame()
    dataset[['text', 'label']] = data.loc[data['Classe'].isin([0, 1, 2]), ['usefull', 'Classe']]
    dataset['label'] = dataset['label'].astype(int)

    # dataset equilibrium
    dataset = dataset.groupby('label').head(100)
    dataset['text'] = dataset['text'].map(lambda x: x.lower())

    # dataset split
    train, test = train_test_split(dataset, test_size=0.2, random_state=42)

    # huggingface dataset
    train = Dataset.from_pandas(train)
    train = train.map(preprocess_function, batched=True)
    test = Dataset.from_pandas(test)
    test = test.map(preprocess_function, batched=True)

    # Load model
    model = AutoModelForSequenceClassification.from_pretrained("dbmdz/bert-base-italian-cased", num_labels=3)
    model.to(device)

    # data collator
    data_collator = DataCollatorWithPadding(tokenizer=tokenizer)

    # training arguments
    training_args = TrainingArguments(
        output_dir='./model_weight',  # output directory
        num_train_epochs=2,  # total number of training epochs
        per_device_train_batch_size=16,  # batch size per device during training
        per_device_eval_batch_size=64,  # batch size for evaluation
        weight_decay=0.01,  # strength of weight decay
        evaluation_strategy="epoch",
        save_strategy="epoch",
        load_best_model_at_end=True,
        metric_for_best_model='eval_accuracy',
        greater_is_better=True,
        report_to="wandb",  # enable logging to W&B
        run_name="bert-base-italian-cased-fit-for-crm",  # name of the W&B run (optional)
        label_names=['0', '1', '2']
    )

    # trainer
    trainer = Trainer(
        model=model,  # the instantiated 🤗 Transformers model to be trained
        args=training_args,  # training arguments, defined above
        train_dataset=train,  # training dataset
        eval_dataset=test,  # evaluation dataset
        data_collator=data_collator,  # data collator
        tokenizer=tokenizer,  # tokenizer
        compute_metrics=compute_metrics,  # the callback that computes metrics of interest
    )

    # train
    trainer.train()
```


### Expected behavior

I expect that with the training function, I can train the network with the tokenizer arguments, and I expect the inference to work when I try to classify a text

Comments:
I have found the solution. The problem is in the Bach dimension for the evaluation that is greater than the number of rows inside the evaluation dataset. Now I have set it to 16 and it works.

But now I have a problem with the inference because I have trained the model with tokenizer max_length settings seated to 1024 and model with the same settings but when I use the model weight for the inference I continue to receive the error about the size of the tensor:

RuntimeError: The size of tensor a (726) must match the size of tensor b (512) at non-singleton dimension 1

Hey! Could you give more details on the exact trace that you are getting? I have no idea where it comes from so can't really help, it could be a problem with loading the checkpoints or anything. 
Also can you share a simple inference reproducing script? Thanks! 

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