Apply `set_execution_file` to specific traces such as forward/backward execution traces

Nobody has claimed this yet.

With https://github.com/Lightning-AI/lightning-thunder/blob/621dce773148e46f4a01a86cbafcb4032eabfcb9/thunder/core/trace.py#L671, we can modify traces before execution, but it seems that every `TraceCtx.python_callable()` call gets affected by the function according to https://github.com/Lightning-AI/lightning-thunder/blob/621dce773148e46f4a01a86cbafcb4032eabfcb9/thunder/core/trace.py#L495-L508.

From my point of view, the number of traces we'd want to modify would be 1 or 2: forward execution trace and/or backward execution trace.
Thus I think it'd be helpful to have an option to only make [`set_execution_callback_file`](https://github.com/Lightning-AI/lightning-thunder/blob/621dce773148e46f4a01a86cbafcb4032eabfcb9/thunder/__init__.py#L225) effective to the two traces mentioned above.

cc @borda @mruberry

Start in thunder/core/trace.py around set_execution_file and TraceCtx.python_callable(), then inspect thunder/__init__.py around set_execution_callback_file. Trace how forward and backward execution traces are created and selected; done means the callback affects only the intended trace or traces without changing other TraceCtx.python_callable() calls.

Written by the indexing model from the issue text.