error with transformers 2.9.1 but not with 2.3.0, same code, why?
# 🐛 Bug

## Information

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

Language I am using the model on (English, Chinese ...): Chinese

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:

My problem is `return encoded` Error with transformers-2.9.1! but with transformers-2.3.0 was fine! Which way is Right? Very Confused!



My code as following:
```
# -*- coding: utf-8 -*-

import torch
from transformers import BertTokenizer, BertModel
from torch.utils.data import Dataset, DataLoader
from functools import partial
import logging

logging.basicConfig(level=logging.INFO)

bert_path = "/Users/kiwi/Desktop/chinese_wwm_ext"
model = BertModel.from_pretrained(bert_path)
tokenizer = BertTokenizer.from_pretrained(bert_path)


def tok_collate(batch_data):
    batch_sentence = [x[0] for x in batch_data]
    encoded = tokenizer.batch_encode_plus(
        batch_sentence,
        add_special_tokens=True,
        return_tensors='pt',
        pad_to_max_length=True)

    #return encoded['input_ids'], encoded['token_type_ids'], encoded['attention_mask']

    # (tensor([[ 101,  704, 1066,  704, 1925, 2600,  741, 6381,  510, 1744, 2157,  712,
    #          2375,  510,  704, 1925, 1092, 1999,  712, 2375,  739, 6818, 2398, 8108,
    #          3189,  683, 7305, 6626, 3959, 1266, 4689, 3636, 3727, 2356, 5440, 2175,
    #          4554, 2658, 7344, 2971, 2339,  868,  102]]), tensor([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    #          0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]]), tensor([[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    #          1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]]))

    return encoded
    #
    #   File "/Users/kiwi/anaconda/python.app/Contents/lib/python3.6/site-packages/transformers/tokenization_utils.py", line 203, in __getattr__
    #     return self.data[item]
    #   KeyError: '__getstate__'



def data_loader(data):
    dl = DataLoader(data, batch_size=8, shuffle=False, collate_fn=partial(tok_collate),
                    num_workers=2)

    for batch_data in dl:
        print(batch_data)


data = [('中共中央总书记、国家主席、中央军委主席习近平10日专门赴湖北省武汉市考察疫情防控工作', 1)]
data_loader(data)

```

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
```
encoded = {'input_ids': tensor([[ 101,  704, 1066,  704, 1925, 2600,  741, 6381,  510, 1744, 2157,  712,
         2375,  510,  704, 1925, 1092, 1999,  712, 2375,  739, 6818, 2398, 8108,
         3189,  683, 7305, 6626, 3959, 1266, 4689, 3636, 3727, 2356, 5440, 2175,
         4554, 2658, 7344, 2971, 2339,  868,  102]]), 'token_type_ids': tensor([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
         0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]]), 'attention_mask': tensor([[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
         1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]])}
```
## 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:2.9.1
- Platform:
- Python version:3.6+
- PyTorch version (GPU?):
- Tensorflow version (GPU?):
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

Hi! Indeed, I can reproduce. This seems to be an edge-case we were not testing for in late v2+ versions. It does run correctly on v2.3.0, as you've shown, and on recent versions (v3+) as well.

After looking a bit deeper into it, it seems to have happened because of the introduction of `BatchEncoding`, in version v2.9.0. It was later patched in v3.0.0 so the blacklisted versions for using a parallelization mechanism (here the dataloader) with `batch_encode_plus` would be the versions between `v2.9.0` and `v3.0.0`. That would be `v2.9.x`, `v2.10.x` and `v2.11.x`.

Hope this helps, and sorry for the inconvenience.