Use no-input nodes by default in converter

[bmorcos]

NengoDL expects a Node with no input connection to receive an input during training/inference even if the node is generating an internal constant. There is a workaround using a lambda output function:

nengo.Node(lambda t, x: 1, size_in=1, label="no-input")

@drasmuss thought it might be nice to make this default behaviour inside the converter for automatically generated networks to avoid some confusion.