The neural network formulations have checks that they're given graphs with single inputs and outputs, but these checks are implemented inconsistently in different places.
the ReluPartitionFormulation checks for single input and output in its '_build_formulation' private method, but that's only ever called by the OmltBlock's 'build_formulation' method, which first checks for single input and output, so the checks in the private method are never hit.
the FullSpaceNetworkFormulation also checks for single input/output on initialization. Maybe the other formulations should also do this, instead of waiting until a check or build time? Or is the expectation that users would have a partially-built network, initialize a formulation on it, and then finalize the network and build?
maybe this validation should be in the base class rather than implemented in each of the different formulations?
The neural network formulations have checks that they're given graphs with single inputs and outputs, but these checks are implemented inconsistently in different places.