Support QUBO with fixed variables

[ZedongPeng]

I am using MQLib to solve a QUBO with fixed variables. However, the following error will come up.
I think we need to add the support of QUBO with fixed variables.

ERROR: LoadError: Constraints of type MathOptInterface.VariableIndex-in-MathOptInterface.EqualTo{Float64} are not supported by the solver.

If you expected the solver to support your problem, you may have an error in your formulation. Otherwise, consider using a different solver.

The list of available solvers, along with the problem types they support, is available at https://jump.dev/JuMP.jl/stable/installation/#Supported-solvers.