1.Fix some bugs:
(1)from_prefix_to_infix,from_postfix_to_infix in mwptoolkit/utils/preprocess_tool/equation_operator.py
(2)the sequence length will be longer than pos_embedder's max length in RobertGen, BertGen.
(3)data preprocessing for new dataset won't automatically remove 'x=' or '=x' in single equation.
2.Update new models:
(1)Seq2Tree model [BertTD]
(2)Seq2Tree model [MWPBert]
3.Rewrite Dataloader and Config
4.Implement function save_dataset() and load_from_pretrained() of Dataset
5.Implement function save_config() and load_from_pretrained() of Config