Add unthunk in conv.jl

[mcabbott]

Aims to fix errors like this in CI, from Zygote 0.7:

AutoDiff: spatial_rank=3: Error During Test at /home/runner/work/Zygote.jl/Zygote.jl/downstream/test/conv.jl:889
  Got exception outside of a @test
  MethodError: no method matching length(::InplaceableThunk{Thunk{ChainRules.var"#719#722"{Float64, Colon, Array{Float64, 5}, ProjectTo{AbstractArray, @NamedTuple{element::ProjectTo{Float64, @NamedTuple{}}, axes::NTuple{5, Base.OneTo{Int64}}}}}}, ChainRules.var"#718#721"{Float64, Colon}})
  The function `length` exists, but no method is defined for this combination of argument types.
  You may need to implement the `length` method or define `IteratorSize` for this type to be `SizeUnknown`.
  
  Closest candidates are:
    length(::JSON.Parser.MemoryParserState)
     @ JSON ~/.julia/packages/JSON/93Ea8/src/Parser.jl:28
    length(::CompositeException)
     @ Base task.jl:51
    length(::LLVM.MemoryBuffer)
     @ LLVM ~/.julia/packages/LLVM/wMjUU/src/buffer.jl:81
    ...
  
  Stacktrace:
    [1] _similar_shape(itr::InplaceableThunk{Thunk{ChainRules.var"#719#722"{Float64, Colon, Array{Float64, 5}, ProjectTo{AbstractArray, @NamedTuple{element::ProjectTo{Float64, @NamedTuple{}}, axes::NTuple{5, Base.OneTo{Int64}}}}}}, ChainRules.var"#718#721"{Float64, Colon}}, ::Base.HasLength)
      @ Base ./array.jl:663
    [2] _collect(cont::UnitRange{Int64}, itr::InplaceableThunk{Thunk{ChainRules.var"#719#722"{Float64, Colon, Array{Float64, 5}, ProjectTo{AbstractArray, @NamedTuple{element::ProjectTo{Float64, @NamedTuple{}}, axes::NTuple{5, Base.OneTo{Int64}}}}}}, ChainRules.var"#718#721"{Float64, Colon}}, ::Base.HasEltype, isz::Base.HasLength)
      @ Base ./array.jl:722
    [3] collect(itr::InplaceableThunk{Thunk{ChainRules.var"#719#722"{Float64, Colon, Array{Float64, 5}, ProjectTo{AbstractArray, @NamedTuple{element::ProjectTo{Float64, @NamedTuple{}}, axes::NTuple{5, Base.OneTo{Int64}}}}}}, ChainRules.var"#718#721"{Float64, Colon}})
      @ Base ./array.jl:716
    [4] colmajor(x::InplaceableThunk{Thunk{ChainRules.var"#719#722"{Float64, Colon, Array{Float64, 5}, ProjectTo{AbstractArray, @NamedTuple{element::ProjectTo{Float64, @NamedTuple{}}, axes::NTuple{5, Base.OneTo{Int64}}}}}}, ChainRules.var"#718#721"{Float64, Colon}})
      @ NNlib ~/work/Zygote.jl/Zygote.jl/downstream/src/conv.jl:339

Closes #619