Use DictHessianPattern as default tracer pattern (#158)

src/interface.jl

@@ -1,6 +1,6 @@
 const DEFAULT_GRADIENT_TRACER = GradientTracer{IndexSetGradientPattern{Int,BitSet}}
 const DEFAULT_HESSIAN_TRACER = HessianTracer{
-    IndexSetHessianPattern{Int,BitSet,Set{Tuple{Int,Int}},NotShared}
+    DictHessianPattern{Int,BitSet,Dict{Int,BitSet},NotShared}
 }
 
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