Make trace introspection available as methods or attribute accesses on functions returned by Thunder

[IvanYashchuk]

🚀 Feature

Make trace introspection available as methods or attribute accesses on functions returned by thunder.jit.

Looking at the last traces of a thunder callable currently requires importing Thunder explicitly. They are currently hidden behind ._lc_cs attribute:

lightning-thunder/thunder/__init__.py

Line 836 in a0a6151

fn_._lc_cs = cs

[t-vi]

We should not add alternative ways to do things much, it is good when there is only one way to achieve things.
What's bad with using thunder.last_traces?

[IvanYashchuk]

There's nothing bad with thunder.last_traces, the alternative is better:

@thunder.jit
def f():
    pass

# Call once ...
f()
# ... and access the traces with
f.last_traces

It doesn't matter in scripts but it's more friendly to REPL development and exploration. Users can hit TAB and see what can be done with this object that thunder.jit returns.
With thunder.last_traces as a free function there's an additional effort involved of knowing and understanding which objects are allowed to be passed to thunder.last_traces.