Obs register fix - #199

#190 introduced a bug into how we create observation backbones in obs_core.py by mistakenly referencing OBS_ENCODER_CORES defined in obs_utils.py. However, we do need a new registry for users to define their own obs encoder backbones such as ResNet18Conv. So here is a fix that does that.

• Changes to utils/obs_utils.py and models/obs_cores.py

  • Added register_encoder_backbone function in robomimic/utils/obs_utils.py to register observation encoder backbones.
  • Introduced OBS_ENCODER_BACKBONES dictionary in robomimic/utils/obs_utils.py to store mappings of observation encoder network names to classes.
  • Updated the backbone_kwargs extraction in robomimic/models/obs_core.py to use OBS_ENCODER_BACKBONES instead of OBS_ENCODER_CORES.

• Class Initialization Updates:

  • Added __init_subclass__ method to ConvBase class in robomimic/models/base_nets.py to automatically register subclasses as valid observation encoders.