Add configuration for XLNet-Base

[huzecong]

The XLNet authors have just released a new pre-trained model checkpoint named XLNet-Base, Cased (https://github.com/zihangdai/xlnet#released-models). We should add support for this checkpoint.

[gpengzhi]

Thank you for the information. I think our current code structure should automatically support this checkpoint.

[huzecong]

@gpengzhi It's not just the checkpoint, but also the configs. We'll need to convert the config JSON file into Texar XLNet hparams, and currently we don't have such functionality.

[huzecong]

We could also take this chance to change everything to batch-first (now XLNet models use the 2nd dimension as the batch dimension). It would be nice to have a unified interface with the rest of the toolkit.

[gpengzhi]

Yes, we are building XLNet modules following a similar way of Bert/GPT2 modules. It will take care of both the checkpoint and the JSON file. @huzecong

[gpengzhi]

@AvinashBukkittu FYI, please change everything to batch-first as well in texar-tensorflow.