The iteration strategy results in a set of job objects with a particular
job index. This method returns the length of that index array when the
specified input types are used. Input types are defined in terms of name
and integer pairs where the name is the name of a NamedInputPortNode in
the iteration strategy and the integer is the depth of the input data
collection (i.e. item depth + index array length for that item which
should be a constant).
inputDepths - map of port names to input collection depth
the length of the index array which will be generated for each
resultant job object.