Document the difference between tracing time and execution time

[tengyifei]

📚 Documentation

If we write a loop like

start = time.time()
for step in range(num_steps):
  run_model()
  xm.mark_step()
end = time.time()

Then end - start will only measure the tracing time. We'll need to do torch_xla.sync(wait=True) to block on device execution to measure the execution time.

We should document this in some "common FAQs/sharp edges" maybe