Add XLNet Modules

[gpengzhi]

1. Add PretrainedBase and modify the code structure of pre-trained modules.
2. Resolve comments in Add XLNet Modules #107
3. Resolve Multiple inheritance with DecoderBase #108
4. Resolve Add configuration for XLNet-Base #106

[huzecong]

Generally LGTM! Since the current solution is working I don't think we need to change the design, but here's an idea I've just had: it might be possible to make PretrainedBase a "mixin" class, i.e. a class that other classes inherit from for the sole purpose of adding a couple of methods and attributes, and whose constructor is not called. If you're familiar with strongly-typed languages, it's a bit like C++/Rust traits and Java interfaces with default implementations.

But anyway, let's keep it as is for now.

[ZhitingHu]

The examples/xlnet example is yet to be simplified, right?

[gpengzhi]

@ZhitingHu Yes. I am working on it.