Add preliminary Metalhead.jl integration

[ablaom]

edited To change syntax from image_builder(constructor)(args...; kwargs...) to image_builder(constructor, args...; kwargs...)

---

This PR replaces #206.

This PR fixes #162 but does so by preparing for Metalhead.jl integration (#205 ) and should be reviewed with that in mind. Full integration is not possible before FluxML/Metalhead.jl#176 is resolved.

Specifically, this PR:

• adds a method image_builder (docstring below)
• adds a temporary proxy VGGHack for Metalhead.VGG as a workaround for Add inchannels, imsize, nclasses as kwargs for all constructors Metalhead.jl#176, which is used as the default builder in ImageClassifier, to resolve Define a working default Flux chain builder for ImageClassifier #162.
• removes MNIST testing as redundant but adds a smoke test for the new default builder

Steps for completing the integration are given in comments in the new code.

Docstring

image_builder(metalhead_constructor, args...; kwargs...)

Return an MLJFlux builder object based on the Metalhead.jl constructor/type
constructor (eg, Metalhead.ResNet). Here args and kwargs are
passed to the MetalheadType constructor at "build time", along with
the extra keyword specifiers imsize=..., inchannels=... and
nclasses=..., with values inferred from the data.

Example

If in Metalhead.jl you would do

using Metalhead
model = ResNet(50, pretrain=true, inchannels=1, nclasses=10)

then in MLJFlux, it suffices to do

using MLJFlux, Metalhead
builder = image_builder(ResNet, 50, pretrain=true)

which can be used in ImageClassifier as in

clf = ImageClassifier(
    builder=builder,
    epochs=500,
    optimiser=Flux.ADAM(0.001),
    loss=Flux.crossentropy,
    batch_size=5,
)

@darsnack Any chance of a review in the next few weeks?

[codecov-commenter]

Codecov Report

Merging #208 (4c8de77) into dev (14b39b1) will decrease coverage by 0.48%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##              dev     #208      +/-   ##
==========================================
- Coverage   93.22%   92.73%   -0.49%     
==========================================
  Files           9       11       +2     
  Lines         251      303      +52     
==========================================
+ Hits          234      281      +47     
- Misses         17       22       +5     

Impacted Files	Coverage Δ
src/builders.jl	100.00% <ø> (ø)
src/mlj_model_interface.jl	94.20% <75.00%> (-2.58%)	⬇️
src/metalhead.jl	89.28% <89.28%> (ø)
src/core.jl	94.73% <100.00%> (ø)
src/types.jl	100.00% <100.00%> (ø)
src/utilities.jl	100.00% <100.00%> (ø)

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[darsnack]

I am not familiar with MLJFlux's code base, but as far as I can see, this looks okay. We can provide the syntax requested in the comments from the Metalhead side.

[ablaom]

We can provide the syntax requested in the comments from the Metalhead side.

That's great. Thanks for taking a look.

[ablaom]

Nightly fail looks unrelated.