correct model selection conditions

[zpyuan6]

Hi, here is minor correction for the model selection between BERT and ALBERT in the model class.

In the previous code, when the model type was entered as "albert-base-v2" or "albert-large-v2", the method would load the corresponding BERT model instead of ALBERT since "bert-base" exists in the "albert-base-v2".

This may explain why the reported ALBERT performance is so close to BERT.