Problems when using PyTorch Class _Dataset_ in model fineturn
Author: Celestite3
Pull request: no

### System Info

```shell
transformer 0.15.1
```


### Information

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

### Tasks

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

### Reproduction

I try to use PyTorch Class _Dataset_ to create my own training task, but it seems make the model worse. After 4 epochs training the model outputs null string. Only  a little part of the input can get right answer. I apprecitate it if someone could save me!!!

```

from datasets import load_dataset
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM
from transformers import DataCollatorForSeq2Seq, Seq2SeqTrainingArguments, Seq2SeqTrainer
import evaluate
import numpy as np
import torch
from torch.utils.data import Dataset

def compute_metrics(eval_preds):
    metric = evaluate.load("sacrebleu")

    preds, labels = eval_preds
    # In case the model returns more than the prediction logits
    if isinstance(preds, tuple):
        preds = preds[0]

    decoded_preds = tokenizer.batch_decode(preds, skip_special_tokens=True)

    # Replace -100s in the labels as we can't decode them
    labels = np.where(labels != -100, labels, tokenizer.pad_token_id)
    decoded_labels = tokenizer.batch_decode(labels, skip_special_tokens=True)

    # Some simple post-processing to remove the "\n", "\t" and so on
    decoded_preds = [pred.strip() for pred in decoded_preds]
    decoded_labels = [[label.strip()] for label in decoded_labels]

    for i in range(10):
        print(decoded_preds[i])
        print(decoded_labels[i])

    result = metric.compute(predictions=decoded_preds, references=decoded_labels)
    return {"bleu": result["score"]}

class MyDataset(Dataset):
    def __init__(self, file_name, tokenizer):
        self.text1 = []
        self.text2 = []
        self.read(file_name)
        self.read(file_name)
        self.encoding = tokenizer(self.text1, text_target=self.text2, truncation=True, max_length=128, padding=True, return_tensors="pt")

    def read(self, file_name):   # Train data is like: "Go.\tVa !"
        with open(file_name, "r", encoding="utf-8") as file:
            while True:
                line = file.readline()
                if line == "":
                    break
                self.text1.append(line.split("\t")[0])
                self.text2.append(line.split("\t")[1])

    def __getitem__(self, index):
        item = {k: v[index].clone().detach() for k, v in self.encoding.items()}
        return item

    def __len__(self):
        return len(self.text1)

def train():
    train_dataset = MyDataset(train_file, tokenizer)
    eval_dataset = MyDataset(eval_file, tokenizer)

    training_args = Seq2SeqTrainingArguments(
        output_dir="save_model",
        learning_rate=2e-5,
        per_device_train_batch_size=8,
        per_device_eval_batch_size=16,
        num_train_epochs=4,
        evaluation_strategy="no",
        save_strategy="epoch",
        save_total_limit=1,
        predict_with_generate=True,
    )

    trainer = Seq2SeqTrainer(
        model=model,
        args=training_args,
        train_dataset=train_dataset,
        eval_dataset=eval_dataset,
        tokenizer=tokenizer,
        compute_metrics=compute_metrics
    )

    print(trainer.evaluate())
    trainer.train()
    print(trainer.evaluate())

if __name__ == "__main__":
    model_name = "t5-small"
    train_file = "fra-eng.txt"
    eval_file = "fra-eng.txt"

    model = AutoModelForSeq2SeqLM.from_pretrained(model_name).to("cuda:0")
    tokenizer = AutoTokenizer.from_pretrained(model_name)

    train()
```


### Expected behavior

```shell
I hope anyone could tell me if I use _Dataset_ Class in a wong way.
```


### Checklist

- [X] I have read the migration guide in the readme. ([pytorch-transformers](https://github.com/huggingface/transformers#migrating-from-pytorch-transformers-to-transformers); [pytorch-pretrained-bert](https://github.com/huggingface/transformers#migrating-from-pytorch-pretrained-bert-to-transformers))
- [X] I checked if a related official extension example runs on my machine.

Comments:
Hi @Cassius31 

This kind of question is better on [Hugging Face Forums](https://discuss.huggingface.co/).

We reserve the `transformers` GitHub repository for issues and feature requests.

> Hi @Cassius31
> 
> This kind of question is better on [Hugging Face Forums](https://discuss.huggingface.co/).
> 
> We reserve the `transformers` GitHub repository for issues and feature requests.

Thanks for reminding! I am new to github and sorry for making trouble. I will delete this 3 hours later.