Bugfix: Export `ConvFeatures`. Required for FastTimm.jl

[RomeoV]

Currently, when following the instructions posted here, using FastTimm.jl fails because FastVision doesn't export ConvFeatures. We fix that here.

(DisentanglingVAE) pkg> precompile
Precompiling environment...
  ✗ FastTimm
  8 dependencies successfully precompiled in 169 seconds. 251 already precompiled.

ERROR: The following 1 direct dependency failed to precompile:

FastTimm [6fe279a3-76fd-4f59-807e-d8e68e6f60cf]

Failed to precompile FastTimm [6fe279a3-76fd-4f59-807e-d8e68e6f60cf] to "/home/romeo/.
julia/compiled/v1.9/FastTimm/jl_bv8rcW".
ERROR: LoadError: UndefVarError: `ConvFeatures` not defined
Stacktrace:
 [1] include
   @ ./Base.jl:456 [inlined]
 [2] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{Str
ing}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{P
air{Base.PkgId, UInt128}}, source::Nothing)
   @ Base ./loading.jl:1952
 [3] top-level scope
   @ stdin:2
in expression starting at /home/romeo/.julia/packages/FastTimm/kKGs7/src/FastTimm.jl:1
in expression starting at stdin:2

By running

$ pwd
# <...>/FastAI.jl/FastVision
$ rg "^struct " src/blocks/

we can check if there's any other blocks we forgot to export, which doesn't seem to be the case.

[lorenzoh]

Thanks for the PR, but I think I would prefer to not have this exported as it's more of an internal Block, much like ImageTensor which isn't exported either. I think qualifying it in FastTimm.jl would be a better solution.

[RomeoV]

No problem. Actually, I see now that this is indeed already imported in FastTimm.jl, so I'm wondering how I got this problem in the first place.

https://github.com/lorenzoh/FastTimm.jl/blob/773f11effcc6ae38c72eac76fbeca3f84f067e12/src/FastTimm.jl#L1-L7