feat: add Initials portion

Author: AayushSabharwal

src/array.jl

@@ -2,6 +2,7 @@ hasportion(::Tunable, ::AbstractArray) = true
 hasportion(::Constants, ::AbstractArray) = false
 hasportion(::Caches, ::AbstractArray) = false
 hasportion(::Discrete, ::AbstractArray) = false
+hasportion(::Initials, ::AbstractArray) = false
 
 struct ArrayRepack{T}
     x::T

src/interface.jl

@@ -111,3 +111,9 @@ struct Discrete <: AbstractPortion end
 The inputs portion of the SciMLStructure.
 """
 struct Input <: AbstractPortion end
+
+"""
+The portion of the SciMLStructure used for parameters solely involved in initialization.
+These should be floating point numbers supporting automatic differentiation.
+"""
+struct Initials <: AbstractPortion end