Flag derivative WRT map itself as not implemented in chain rule (#198)

Author: gaurav-arya

src/LinearMaps.jl

@@ -11,7 +11,7 @@ using SparseArrays
 
 import Statistics: mean
 
-using ChainRulesCore: unthunk, NoTangent, @thunk
+using ChainRulesCore: unthunk, NoTangent, @thunk, @not_implemented
 import ChainRulesCore: rrule
 
 using Base: require_one_based_indexing

src/chainrules.jl

@@ -2,8 +2,7 @@ function rrule(::typeof(*), A::LinearMap, x::AbstractVector)
     y = A*x
     function pullback(dy)
         DY = unthunk(dy)
-        # Because A is an abstract map, the product is only differentiable w.r.t the input
-        return NoTangent(), NoTangent(), @thunk(A' * DY)
+        return NoTangent(), @not_implemented("Gradient with respect to linear map itself not implemented."), @thunk(A' * DY)
     end
     return y, pullback
 end
@@ -12,8 +11,7 @@ function rrule(A::LinearMap, x::AbstractVector)
     y = A*x
     function pullback(dy)
         DY = unthunk(dy)
-        # Because A is an abstract map, the product is only differentiable w.r.t the input
-        return NoTangent(), @thunk(A' * DY)
+        return @not_implemented("Gradient with respect to linear map itself not implemented."), @thunk(A' * DY)
     end
     return y, pullback
 end