character tokenizer using BertTokenizer (uncased)
from transformers import AutoTokenizer, BertForMaskedLM, BertConfig
MODEL_NAME = 'char-bert-base-uncased'
tokenizer = AutoTokenizer.from_pretrained(MODEL_NAME, use_fast=True)
config = BertConfig(vocab_size=len(tokenizer))
model = BertForMaskedLM(config)