Add CPU option for transformer models

[Mirochill]

Summary

• Add a --cpu option that keeps local transformers models off CUDA even when PyTorch detects a GPU.
• Thread the CPU/CUDA choice through preset and custom transformer model construction.
• Document the option for incompatible CUDA/PyTorch environments.

Fixes #57.

Validation

• git diff --check HEAD~1..HEAD

Tests not run locally.