Config fix

[agoryuno]

I've rewritten the torchscale.architecture.config module to use inheritance and remove the redundant code. There are now 3 classes:

Config - that holds all common options
EncoderConfig - inherits 'Config' and holds Encoder specific options
DecoderConfig - inherits 'Config' and holds Decoder specific options
EncoderDecoderConfig - inherits EncoderConfig and DecoderConfig and holds that one option shared by Encoder and Decoder objects.

I also took the liberty to add some type hints for args parameters in encoder/decoder modules.