Encode special tokens

[ArthurZucker]

Allow skipping special tokens when encoding

fixes #1347, fixes #1391 fixes #1368

Snippet:

>>> from transformers import AutoTokenizer
>>> tokenizer = AutoTokenizer.from_pretrained("gp2") 
>>> tokenizer.tokenize("<|endoftext|>")
['<|endoftext|>']

>>> tokenizer._tokenizer.encode_special_tokens = True
>>> tokenizer.tokenize("<|endoftext|>")
['<', '|', 'end', 'of', 'text', '|', '>']

[HuggingFaceDocBuilderDev]

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