add abstract types for models and kernels

Author: yuehhua

src/DeepONet/DeepONet.jl

@@ -81,7 +81,7 @@ branch net: (Chain(Dense(2, 128), Dense(128, 64), Dense(64, 72)))
 Trunk net: (Chain(Dense(1, 24), Dense(24, 72)))
 ```
 """
-struct DeepONet{T1, T2}
+struct DeepONet{T1, T2} <: AbstractOperatorModel
     branch_net::T1
     trunk_net::T2
 end

src/NOMAD/NOMAD.jl

@@ -1,6 +1,6 @@
 export NOMAD
 
-struct NOMAD{T1, T2}
+struct NOMAD{T1, T2} <: AbstractOperatorModel
     approximator_net::T1
     decoder_net::T2
 end

src/NeuralOperators.jl

@@ -11,6 +11,8 @@ using ChainRulesCore
 using GeometricFlux
 using Statistics
 
+include("abstracttypes.jl")
+
 # kernels
 include("Transform/Transform.jl")
 include("operator_kernel.jl")

src/abstracttypes.jl

@@ -0,0 +1,6 @@
+export
+    AbstractOperatorModel,
+    AbstractOperatorKernel
+
+abstract type AbstractOperatorModel end
+abstract type AbstractOperatorKernel end

src/operator_kernel.jl

@@ -113,7 +113,7 @@ end
 # operator #
 ############
 
-struct OperatorKernel{L, C, F}
+struct OperatorKernel{L, C, F} <: AbstractOperatorKernel
     linear::L
     conv::C
     σ::F