Support for more complex operations on YAML config files, working off YACS

[ozan-oktay]

YACS Features: https://detectron2.readthedocs.io/en/latest/_modules/yacs/config.html

• YAML based config objects.
• Mutable/Immutable flags.
• Supports tree-hierarchy for objects - e.g., config -> config.model and config.training -> config.model.img_model.
• Basic type checking through type interpretation - e.g., it fails when variables are specified as None.
• Supports command line overrides - https://github.com/rbgirshick/yacs

TODO List:

• Does it support config inheritance?
• We need to try yacs._load_cfg_py_source function to see if it can work with python files instead of yaml files for the ease of unit-testing and potentially inheritance.
• Check if there are any other alternative libraries that one could use.

[ant0nsc]

Idea for inheritance:

TEMPLATE: ../foo/bar.yml
SYSTEM:
  NUM_GPUS: 2
TRAIN:
  SCALES: (1, 2)

this should allow multiple levels of inheritance

[ant0nsc]

templates could also be added for specific parts of the tree

[ant0nsc]

Allow type checking on fields that are None by default. Could use params library, or do introspection to retrieve the type annotation