tokenizer's is_split_into_words seems not work
Author: HenryPaik1
Pull request: no
Labels: name: New model

I input tokenized list of tokens, but it return different result(not count pad token). It seems tokenize pretokenized tokens, ignoring `is_split_into_words`. Please refer to the code below:

```
sent = "the latest investigation was authorized after the supreme court in 2007 found dcc and its founder , jim flavin , guilty of selling dcc 's ( euro ) 106 million ( then $ 130 million ) stake in fyffes after flavin -- also a fyffes director at the time -- received inside information about bad fyffes news in the pipeline ."

encoded_dict = tokenizer.encode_plus(
                sent,       # Sentence to encode.
                add_special_tokens = False, # Add '[CLS]' and '[SEP]'
                max_length = 314,           # Pad & truncate all sentences.
                padding = 'max_length',
                return_attention_mask = True,   # Construct attn. masks.
                return_tensors = 'pt', 
                return_token_type_ids=False,# Return pytorch tensors.
                truncation=False,
                is_split_into_words=False)
input_ids = encoded_dict['input_ids']
tokenized = tokenizer.convert_ids_to_tokens([i.item() for i in input_ids.squeeze() if i > 1])
len(tokenized)
>> 79

print(tokenized)
>> ['the', 'latest', 'investigation', 'was', 'authorized', 'after', 'the', 'supreme', 'court', 'in', '2007', 'found', 'dc', '##c', 'and', 'its', 'founder', ',', 'jim', 'fl', '##avi', '##n', ',', 'guilty', 'of', 'selling', 'dc', '##c', "'", 's', '(', 'euro', ')', '106', 'million', '(', 'then', '$', '130', 'million', ')', 'stake', 'in', 'f', '##y', '##ffe', '##s', 'after', 'fl', '##avi', '##n', '-', '-', 'also', 'a', 'f', '##y', '##ffe', '##s', 'director', 'at', 'the', 'time', '-', '-', 'received', 'inside', 'information', 'about', 'bad', 'f', '##y', '##ffe', '##s', 'news', 'in', 'the', 'pipeline', '.']

###### tokenizing pretokenized tokens as list
encoded_dict = tokenizer.encode_plus(
                tokenized,       # Sentence to encode.
                add_special_tokens = False, # Add '[CLS]' and '[SEP]'
                max_length = 314,           # Pad & truncate all sentences.
                padding = 'max_length',
                return_attention_mask = True,   # Construct attn. masks.
                return_tensors = 'pt', 
                return_token_type_ids=False,# Return pytorch tensors.
                truncation=False,
                is_split_into_words=True)

input_ids = encoded_dict['input_ids']
tokenized = tokenizer.convert_ids_to_tokens([i.item() for i in input_ids.squeeze() if i > 1])
len(tokenized)
>> 114 # it should be 79

print(tokenized)
>> ['the', 'latest', 'investigation', 'was', 'authorized', 'after', 'the', 'supreme', 'court', 'in', '2007', 'found', 'dc', '#', '#', 'c', 'and', 'its', 'founder', ',', 'jim', 'fl', '#', '#', 'av', '##i', '#', '#', 'n', ',', 'guilty', 'of', 'selling', 'dc', '#', '#', 'c', "'", 's', '(', 'euro', ')', '106', 'million', '(', 'then', '$', '130', 'million', ')', 'stake', 'in', 'f', '#', '#', 'y', '#', '#', 'ff', '##e', '#', '#', 's', 'after', 'fl', '#', '#', 'av', '##i', '#', '#', 'n', '-', '-', 'also', 'a', 'f', '#', '#', 'y', '#', '#', 'ff', '##e', '#', '#', 's', 'director', 'at', 'the', 'time', '-', '-', 'received', 'inside', 'information', 'about', 'bad', 'f', '#', '#', 'y', '#', '#', 'ff', '##e', '#', '#', 's', 'news', 'in', 'the', 'pipeline', '.']
```

Comments:
wrongly posted. I delete it.