Need `mode` method for SecondOrder

[ElOceanografo]

I was trying to compare some second-order methods for Hessians using DifferentiationInterfaceTest, but found there is no mode method defined for SecondOrder backends:

julia> DifferentiationInterface.mode(SecondOrder(AutoSparseForwardDiff(), AutoReverseDiff()))
ERROR: MethodError: no method matching mode(::SecondOrder{AutoSparseForwardDiff{nothing, Nothing}, AutoReverseDiff})

Closest candidates are:
  mode(::AutoChainRules{<:ChainRulesCore.RuleConfig{>:ChainRulesCore.HasReverseMode}})
   @ DifferentiationInterfaceChainRulesCoreExt ~/.julia/packages/DifferentiationInterface/9POaB/ext/DifferentiationInterfaceChainRulesCoreExt/DifferentiationInterfaceChainRulesCoreExt.jl:17
  mode(::AutoChainRules{<:ChainRulesCore.RuleConfig{>:ChainRulesCore.HasForwardsMode}})
   @ DifferentiationInterfaceChainRulesCoreExt ~/.julia/packages/DifferentiationInterface/9POaB/ext/DifferentiationInterfaceChainRulesCoreExt/DifferentiationInterfaceChainRulesCoreExt.jl:16
  mode(::Union{AutoSparseFiniteDiff, AutoSparseZygote, AutoSparseFastDifferentiation, AutoSparseSymbolics, AutoSparseReverseDiff, AutoSparseForwardDiff, AutoSparsePolyesterForwardDiff})
   @ DifferentiationInterface ~/.julia/packages/DifferentiationInterface/9POaB/src/sparse.jl:37
  ...

Stacktrace:
 [1] top-level scope
   @ REPL[5]:1

...which causes an error when creating the table of benchmark results.

[gdalle]

Good catch! I'll add this fix to the main branch so you can use it, but it won't be registered until next week or so because v0.3 is a big revamp