Respect `MOI.AutomaticDifferentiationBackend()` in NLP

[klamike]

The current code has SparseReverseMode hard-coded:

DiffOpt.jl/src/NonLinearProgram/nlp_utilities.jl

Lines 94 to 109 in bf2c4c3

	"""
	_create_evaluator(form::Form)
	Create the evaluator for the NLP.
	"""
	function _create_evaluator(form::Form)
	nlp = form.model
	backend = MOI.Nonlinear.SparseReverseMode()
	evaluator = MOI.Nonlinear.Evaluator(
	nlp,
	backend,
	MOI.VariableIndex.(1:form.num_variables),
	)
	MOI.initialize(evaluator, [:Hess, :Jac])
	return evaluator
	end

Perhaps backend should be set based on MOI.AutomaticDifferentiationBackend()? Besides the SymbolicMode, it would also allow to use DiffOpt with custom evaluators.

[joaquimg]

We do need an attribute for setting that.

[blegat]

We could use MOI.AutomaticDifferentiationBackend by default. We could have a different attribute in case the user wants a different AD than the solver but using the same as the solver to start with would be a good first step