Add transfer learning code with no GPU

[logankilpatrick]

Unless the CPU code is un-runnable as is, we should give a CPU only option here: https://fluxml.ai/tutorials/2020/10/18/transfer-learning.html

[ToucheSir]

gpu is a no-op when CUDA.jl is disabled, so this example should run as-is. That said, we could always add a code snippet or a couple sentences that talk about this.