Implement CORN loss for logit outputs

[gmgeorg]

The coral-pytorch authors recently published an update on CORAL ordinal regression, called CORN ordinal regression. tl;dr: it's a stick breaking process implementation of ordinal regression, which means no weight constraints are necessary to keep rank consistency.

See here for template
https://github.com/Raschka-research-group/coral-pytorch/blob/main/coral_pytorch/losses.py#L86

TF keras implementation can be completely analogous using tf.math.log_sigmoid().