GPT2Tokenizer.add_tokens() didnt change tokenizer.vocab_size
follow the instruction in add_tokens()

code:

special_tokens = ['<pad>', '<go_r>', '<unk>', '<go_b>', '<go_a>', '<go_u>', ]
num_added_toks = tokenizer.add_tokens(special_tokens)
print('We have added', num_added_toks, 'tokens')

it can successfully encode and decode the newly added tokens, but the GPT2Tokenizer.vocab_size is still 50257. 
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