Add :spec_overrides to Bumblebee.load_model/2

[jonatanklosko]

{:ok, spec} = Bumblebee.load_spec({:hf, "microsoft/resnet-50"})
spec = Bumblebee.configure(spec, num_labels: 10)
{:ok, resnet} = Bumblebee.load_model({:hf, "microsoft/resnet-50"}, spec: spec)

->

{:ok, resnet} =
  Bumblebee.load_model({:hf, "microsoft/resnet-50"}, spec_overrides: [num_labels: 10])

I considered accepting a keyword list for :spec, but probably better to kept them separate for clarity.