Fix expand_derivatives

Author: YingboMa

src/differentials.jl

@@ -57,7 +57,6 @@ function expand_derivatives(O::Operation)
 
     return O
 end
-expand_derivatives(::Constant) = Constant(0)
 expand_derivatives(x) = x
 
 # Don't specialize on the function here
@@ -96,7 +95,7 @@ sin(x())
 ```
 """
 derivative(O::Operation, idx) = derivative(O.op, (O.args...,), Val(idx))
-derivative(O::Constant, _) = Constant(0)
+derivative(O::Constant, ::Any) = Constant(0)
 
 # Pre-defined derivatives
 import DiffRules, SpecialFunctions, NaNMath