Installation verification tool

When installing the S4TF toolchain, it's not always clear whether all components are intact and versions are compatible. It would be helpful to have a quick verification tool that uses the toolchain and reports success.

This is especially useful for installations involving accelerators, so the first two features could be:

• Can invoke the toolchain and import TensorFlow
• Can run on an attached accelerator (Device.defaultXLA is Device(kind: .GPU, ordinal: 0, backend: .XLA))

This could be a very basic Swift tool that automates the steps we've been taking manually.