🚀 Feature

Offer the option to set box constraints for optimizers.

Motivation

In some applications of reinforcement learning, falling outside of a region correspond to inadmissible actions.

Alternatives

One can omit the constraints, and hope that the optimal parameters found happen to be within the constraints, without any guarantees.

Additional context

This was suggested in #938 for torch.optim.lbfgs.

CC @bamos