[tinker-cookbook] supervised: add tracing annotations

[kennyyu]

This adds tracing annotations to make it easier to identify where time is spent in SFT workloads. To run it:

• set enable_trace=True
• Run sl_basic:

python -m tinker_cookbook.recipes.sl_basic enable_trace=true

• While the job is running, generate the trace:

python -m tinker_cookbook.utils.trace /tmp/tinker-examples/sl_basic/trace_events.jsonl /tmp/sl_basic.json

• And then visualize with perfetto:

[joschu]

nit: could be nice to have a helper to do
context.set_attribute('step', step)
or even
set_scope_attribute('step', step)
which does the get_scope_context() internally

[kennyyu]

Pre-existing type check/uv errors