Relax AbstractOptimizer requirement in instantiate

[odow]

Alternative to #1780

This will need JuMP to lower-bound it's dependence on MOI, but that's okay. This is really a bug-fix, so I plan to tag as 1.1.1.

julia> using MathOptInterface; const MOI = MathOptInterface
MathOptInterface

julia> MOI.instantiate(MOI.FileFormats.NL.Model)
An AMPL (.nl) model

julia> MOI.instantiate(MOI.FileFormats.NL.Model; with_bridge_type = Float64)
MOIB.LazyBridgeOptimizer{MOIU.CachingOptimizer{MOI.FileFormats.NL.Model, MOIU.UniversalFallback{MOIU.Model{Float64}}}}
with 0 variable bridges
with 0 constraint bridges
with 0 objective bridges
with inner model MOIU.CachingOptimizer{MOI.FileFormats.NL.Model, MOIU.UniversalFallback{MOIU.Model{Float64}}}
  in state EMPTY_OPTIMIZER
  in mode AUTOMATIC
  with model cache MOIU.UniversalFallback{MOIU.Model{Float64}}
    fallback for MOIU.Model{Float64}
  with optimizer An AMPL (.nl) model

julia> MOI.instantiate(MOI.FileFormats.MPS.Model; with_bridge_type = Float64)
MOIB.LazyBridgeOptimizer{MOIU.GenericModel{Float64, MOIU.ObjectiveContainer{Float64}, MOIU.VariablesContainer{Float64}, MOI.FileFormats.MPS.ModelFunctionConstraints{Float64}}}
with 0 variable bridges
with 0 constraint bridges
with 0 objective bridges
with inner model A Mathematical Programming System (MPS) model

[blegat]

This will need JuMP to lower-bound it's dependence on MOI

I still don't understand why this would while the other fix wouldn't since none of them require a change in JuMP so what's the difference between the two fixes that I am missing ?

[odow]

JuMP doesn't use instantiate:
https://github.com/jump-dev/JuMP.jl/blob/7fdd4d41f5275f2f5a0775f5d89252c689128413/src/file_formats.jl#L51-L55
So when we switch to MOI.instantiate(MOI.FileFormats.XXX.Model) it won't work on an earlier version of MOI

[odow]

This does need a change in JuMP. #1780 fixes it on the MOI side, so it doesn't need a change in JuMP.