Can't pickle tokenizers ...
Tokenizers are losing all their special tokens when un-pickled.  Not sure if there are other attributes that aren't being rehydrated as well ...

```
hf_tokenizer
# <transformers.tokenization_roberta.RobertaTokenizer at 0x7fc4ce625f10>

hf_tokenizer.special_tokens_map
# {'bos_token': '<s>',
# 'eos_token': '</s>',
# 'unk_token': '<unk>',
# 'sep_token': '</s>',
# 'pad_token': '<pad>',
# 'cls_token': '<s>',
# 'mask_token': '<mask>'}

pickle.dump( hf_tokenizer, open( "save.p", "wb" ) )
hf_tokenizer = pickle.load( open( "save.p", "rb" ) )

hf_tokenizer.special_tokens_map
# {'bos_token': '',
# 'eos_token': '',
# 'unk_token': '',
# 'sep_token': '',
# 'pad_token': '',
# 'cls_token': '',
# 'mask_token': ''}

```
Hi, ok I can reproduce, this is a bug in the `AddedToken`class of `huggingface/tokenizers`. Moving this up.