Nonlinear constraints are limited to single output functions

[khurram-ghani]

The NonlinearConstraint class currently only supports constraint functions that return a single output. The user can still provide a list of constraints in the search space, but each nonlinear instance can contain only a single constraint. Even though the trieste code itself supports multi-output constraints, the tfp.math.value_and_gradient call used in NonlinearConstraint.__init__ does not work correctly. It returns the sum of gradients instead of the Jacobian needed. It will likely require a custom gradient implementation to support this correctly.