Fix bumblebee detection

[mat-hek]

Currently, the Image.Classification module is loaded when Bumblebee, Nx and Exla are available. However, Image.Application calls Image.Classification when Bumblebee is available. So, if one has Bumblebee in deps, but not Exla, this error is raised:

** (Mix) Could not start application image: exited in: Image.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (UndefinedFunctionError) function Image.Classification.classifier/0 is undefined (module Image.Classification is not available)
            Image.Classification.classifier()
            (image 0.54.1) lib/application.ex:34: anonymous fn/2 in Image.Application.children/1
            (elixir 1.17.2) lib/enum.ex:2531: Enum."-reduce/3-lists^foldl/2-0-"/3
            (image 0.54.1) lib/application.ex:18: Image.Application.start/2
            (kernel 10.0) application_master.erl:295: :application_master.start_it_old/4

Even though using Bumblebee without Exla may make little sense, this PR aims to fix that by calling Image.bumblebee_configured? in both Application and Classification ;)

[kipcole9]

@mat-hek, thank much and greatfully accepted.