batch_encode_plus cannot work properly
I have data like below:
```
>>> ds[0][0:3]
["John was writing lyrics for his new album.He started experiencing writer 's block.He tried to force himself to write but it would n't do anythingHe tried to force himself to write but it would n't do anything.He took a walk , hung out with some friends , and looked at natureHe took a walk , hung out with some friends , and looked at natureHe took a walk , hung out with some friends , and looked at nature", 'Franny did not particularly like all of the immigration happening.She thought immigrants were coming to cause social problemsShe thought immigrants were coming to cause social problems.Franny was upset when an immigrant moved in next door.The immigrant , Sal , was kind and became friends with FrannyThe immigrant , Sal , was kind and became friends with Franny', 'Ari spends $ 20 a day on pickles.He decides to make his own to save money.He puts the pickles in brine.Ari waits 2 weeks for his pickles to get sour']
>>> ds[1][0:3]
['He felt inspiration and then went back home to write', 'When he finished his paper he went to bedWhen he finished his paper he went to bed', 'Trudey hoped self-publishing would be more profitable']
```
I was trying to do next sentence prediction using BERT model, it is necessary to use text pair to finish this problem.However,when I was tring to encode ds[0] and ds[1] as a batched text pair,I have met following problem.
```
>>> input_wsrq1ids=tokenizer.batch_encode_plus([(ds[0][0:3],ds[1][0:3])],add_special_tokens=True,return_tensors='pt')
>>> input_wsrq1ids
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'inp_wsrq1ids' is not defined
>>> input_wsrq1ids
{'input_ids': tensor([[101, 100, 100, 100, 102, 100, 100, 100, 102]]), 'token_type_ids': tensor([[0, 0, 0, 0, 0, 1, 1, 1, 1]]), 'attention_mask': tensor([[1, 1, 1, 1, 1, 1, 1, 1, 1]])}
```
The answer is definitely not the right encoding fot this data. I did not quite understand this documentation of batch_encod_plus method,it says for text_pair,look for encode_plus for details.However,the encode_plus only works for non-batched data, and there is no clue and example code to show us how to use batch_enocde_plus properly.
This is quite a weird way to encode your data with `ds[0][0:3]`. There are two cases:

1) You have a single string you want to encode:
```
input_str = 'hello, what time is it?'
input_ids_dict = tokenizer.encode_plus(input_str)
```

2) You have a batch of input string data that you want to encode:

```
input_str_batch = ['hello what time is it' , 'hello, how are you?', 'Hey, I'm Peter']
input_ids_dict = tokenizer.batch_encode_plus(input_str_batch, pad_to_max_length=True)
```

Also see #3237