"Sequence Classification with IMDb Reviews " error, when using "bert-base-multilingual-cased" model.
Author: baiziyuandyufei
Pull request: no

## 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: 3.1.0
- Platform:macos
- Python version:3.6
- PyTorch version (GPU?):CPU
- Tensorflow version (GPU?):CPU
- Using GPU in script?:No
- Using distributed or parallel set-up in script?:No

### Who can help
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 albert, bert, GPT2, XLM: @LysandreJik 
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Speed and Memory Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 Translation: @sshleifer
 Summarization: @sshleifer
 TextGeneration: @TevenLeScao 
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @TevenLeScao
 blenderbot: @mariamabarham
 Bart: @sshleifer
 Marian: @sshleifer
 T5: @patrickvonplaten
 Longformer/Reformer: @patrickvonplaten
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 examples/seq2seq: @sshleifer
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
 documentation: @sgugger
 -->

## Information

Model I am using (Bert, XLNet ...):

The problem arises when using:
* [+] the official example scripts: (give details below)
* [+] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1.  reference the code [https://huggingface.co/transformers/custom_datasets.html#seq-imdb](url)
2. modify the code 
```
# coding:utf-8
"""
"""

from pathlib import Path
from sklearn.model_selection import train_test_split
from transformers import DistilBertTokenizerFast
import torch
from transformers import Trainer, TrainingArguments
from nlp import load_dataset
from transformers import AutoTokenizer, AutoModelWithLMHead


tokenizer = AutoTokenizer.from_pretrained("bert-base-multilingual-cased")  
model = AutoModelWithLMHead.from_pretrained("bert-base-multilingual-cased")


def read_imdb_split(split_dir):
    split_dir = Path(split_dir)
    texts = []
    labels = []
    for label_dir in ["pos", "neg"]:
        for text_file in (split_dir/label_dir).iterdir():
            texts.append(text_file.read_text())
            labels.append(0 if label_dir is "neg" else 1)

    return texts, labels

train_texts, train_labels = read_imdb_split('dataset/aclImdb/train')
test_texts, test_labels = read_imdb_split('dataset/aclImdb/test')
train_texts, val_texts, train_labels, val_labels = train_test_split(train_texts, train_labels, test_size=.2)


train_encodings = tokenizer(train_texts, truncation=True, padding=True, max_length=100)
val_encodings = tokenizer(val_texts, truncation=True, padding=True, max_length=100)
test_encodings = tokenizer(test_texts, truncation=True, padding=True, max_length=100)



class IMDbDataset(torch.utils.data.Dataset):
    def __init__(self, encodings, labels):
        self.encodings = encodings
        self.labels = labels

    def __getitem__(self, idx):
        item = {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}
        item['labels'] = torch.tensor(self.labels[idx])
        return item

    def __len__(self):
        return len(self.labels)

train_dataset = IMDbDataset(train_encodings, train_labels)
val_dataset = IMDbDataset(val_encodings, val_labels)
test_dataset = IMDbDataset(test_encodings, test_labels)


training_args = TrainingArguments(
    output_dir='./results',
    num_train_epochs=1,
    per_device_train_batch_size=16,
    per_device_eval_batch_size=64,
    warmup_steps=500,
    weight_decay=0.01,
    evaluate_during_training=True,
    logging_dir='./logs',
)

trainer = Trainer(
    model=model,
    args=training_args,
    train_dataset=train_dataset,
    eval_dataset=test_dataset
)

trainer.train()
```
3. the error info
```
ValueError: Expected input batch_size (1600) to match target batch_size (16).
```

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->


Comments:
It looks like you are using a model for language modeling (`AutoModelWithLMHead`) instead of a model for sequence classification (`AutoModelForSequenceClassification`) which is why you have that shape error.

@sgugger thank you!
I modify my code, then everything well.
```
# coding:utf-8
"""
"""

from pathlib import Path
from sklearn.model_selection import train_test_split
import torch
from transformers import Trainer, TrainingArguments
from nlp import load_dataset
from transformers import AutoTokenizer, AutoModelForSequenceClassification


tokenizer = AutoTokenizer.from_pretrained("model/bert-base-multilingual-cased")  
model = AutoModelForSequenceClassification.from_pretrained("model/bert-base-multilingual-cased")


def read_imdb_split(split_dir):
    split_dir = Path(split_dir)
    texts = []
    labels = []
    for label_dir in ["pos", "neg"]:
        for text_file in (split_dir/label_dir).iterdir():
            texts.append(text_file.read_text())
            labels.append(0 if label_dir is "neg" else 1)

    return texts, labels

train_texts, train_labels = read_imdb_split('dataset/ChnSentiCorp')
test_texts, test_labels = read_imdb_split('dataset/ChnSentiCorp')
train_texts, val_texts, train_labels, val_labels = train_test_split(train_texts, train_labels, test_size=.2)


train_encodings = tokenizer(train_texts, truncation=True, padding=True, max_length=100, verbose=False)
val_encodings = tokenizer(val_texts, truncation=True, padding=True, max_length=100, verbose=False)
test_encodings = tokenizer(test_texts, truncation=True, padding=True, max_length=100, verbose=False)



class IMDbDataset(torch.utils.data.Dataset):
    def __init__(self, encodings, labels):
        self.encodings = encodings
        self.labels = labels

    def __getitem__(self, idx):
        item = {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}
        item['labels'] = torch.tensor(self.labels[idx])
        return item

    def __len__(self):
        return len(self.labels)

train_dataset = IMDbDataset(train_encodings, train_labels)
val_dataset = IMDbDataset(val_encodings, val_labels)
test_dataset = IMDbDataset(test_encodings, test_labels)


training_args = TrainingArguments(
    output_dir='./results',
    num_train_epochs=16,
    per_device_train_batch_size=1,
    per_device_eval_batch_size=1,
    warmup_steps=500,
    weight_decay=0.01,
    evaluate_during_training=True,
    logging_dir='./logs',
)

trainer = Trainer(
    model=model,
    args=training_args,
    train_dataset=train_dataset,
    eval_dataset=test_dataset
)

trainer.train()
```

```
Epoch:   0%|                                                                              | 0/16 [00:00<?, ?it/s]
Iteration:   2%|█▏                                                           | 65/3200 [03:04<2:26:14,  2.80s/it]
```