Blueprint Mixin cleanup and test

[JackUrb]

Overview

Finishes formalizing the content for BlueprintMixins, as well as the methodology for using them. With this change, we address two types of mixin users:

1. People who want to simply include a mixin in a simple blueprint, but don't need to be creating their own BlueprintArgs, SharedTaskState, etc.
2. People who are building a complicated blueprint, and likely making their own of the previous listed classes.

It then includes machinery to discover Mixins for a blueprint, and automatically call their initializers and asserts. It also includes fixes for the ParlAI blueprint, and testing for the mixin architecture.

Implementation

Rather than using metaclass magic, in order to address both of the above classes, simple case users can use the @BlueprintMixin. mixin_args_and_state decorator on their Blueprint class, for whatever BlueprintMixins they want. Our tasks (ParlAIChatBlueprint) use the extended form, in which we ask each configuration class to also mixin the mixin's defined configuration classes.

This method wraps the created Blueprint with a MixedInBlueprint which automatically incorporates and mixes in the config mixins that are required. Generally it's more opaque than doing it yourself, but it's certainly simpler for people who don't need to get into the nitty-gritty of defining the arguments.

Discussion

The extract_unique_mixins method is likely the most "magic" method in this, going through the class structure of a given Blueprint and discovering all BlueprintMixins that can be considered "unique" (not directly subclassed by another mixin that is also included). I think this covers most cases to avoid redundancy, but in any case the init_mixin_config method of any BlueprintMixin should be idempotent to deal with cases where there are strange inheritance structures.

@kushalchawla this should resolve your issue, and you can return to using main once it is merged. Sorry for the inconvenience!

Testing

Added new testing, existing tests pass after the changes that move mixin config inits into the Blueprint base class.

[pringshia]

Thanks for the tests, they make it easier to follow along what's happening here. I think the decorator is a nice, clean touch. On my end it looks good. Perhaps someone with more Python expertise can chime in as well, but on my end I'm ok to approve!