We discussed that generating formulations that introduce MIP constraints for ReLU and non-linear constraints for other activations should not be possible. OMLT should throw and exception in this case.
The new formulations as of PR #33 should fix this. ReluBigMFormulation will not allow nonlinear activation functions, but a user can certainly try to mix activations using FullspaceNNFormulation.
We discussed that generating formulations that introduce MIP constraints for ReLU and non-linear constraints for other activations should not be possible. OMLT should throw and exception in this case.