Creates the nn_graph from a ModelDescriptor. Mostly for internal use, since the ModelDescriptor is in most circumstances harder to use than just creating nn_graph directly.
Model Descriptor. pointer is ignored, instead output_pointer values are used. $graph member is modified by-reference.
output_pointers: (list of character)
Collection of pointers that indicate what part of the model_descriptor$graph is being used for output. Entries have the format of ModelDescriptor$pointer.
list_output: (logical(1))
Whether output should be a list of tensors. If FALSE, then length(output_pointers) must be 1.