refactor(examples) Update `advanced-pytorch` example

[jafermarq]

Updated example showing:

• custom strategy that:
  • [x] saves model checkpoint when a new best is found
  • [x] saves all metrics to the file system as JSON
  • [x] logs metrics to weight and biases (if enabled)
  • [x] Include LR decay from strategy to clients
• [x] Demonstrates how to implement stateful clients
• [x] test on GPU