[deep1blue]

What does this PR do?

Fixes two self-discovered profiler edge-case bugs in lightning.pytorch.profilers:

1. AdvancedProfiler.__reduce__ silently resets dump_stats=True to False (src/lightning/pytorch/profilers/advanced.py):

   • AdvancedProfiler.__reduce__ returns (self.__class__, (), {...}) so cProfile.Profile objects do not raise TypeError during pickling/deepcopy (e.g., when cloning the trainer/callbacks or spawning worker processes).
   • Because __init__ is invoked with () (where dump_stats=False by default) and the returned state dictionary omitted "dump_stats": self.dump_stats, any copy.deepcopy(profiler) or pickle.loads(pickle.dumps(profiler)) silently reset profiler.dump_stats from True back to False, preventing .prof files from being written in describe().
   • Adding "dump_stats": self.dump_stats to the __reduce__ state dictionary preserves dump_stats across pickling and deepcopy.

2. SimpleProfiler(extended=False).summary() raises ValueError: max() iterable argument is empty when no actions have been recorded (src/lightning/pytorch/profilers/simple.py):

   • In SimpleProfiler.summary(), the if self.extended: branch guards max(len(k) for k in self.recorded_durations) with if len(self.recorded_durations) > 0:, returning an empty report header when called before any actions are recorded.
   • However, the else: branch (extended=False) omitted that guard and unconditionally called max(len(k) for k in self.recorded_durations), raising ValueError: max() iterable argument is empty if summary() or describe() is called on an empty SimpleProfiler(extended=False).
   • Replacing else: with elif len(self.recorded_durations) > 0: makes SimpleProfiler(extended=False) consistent with SimpleProfiler(extended=True).

Testing

pytest tests/tests_pytorch/profilers/test_profiler.py -k "simple_profiler or advanced_profiler" -v
# 37 passed, 1 skipped

Acknowledgements

Credit to the Lightning AI team and contributors (@Borda, @carmocca, @awaelchli, @ajstewart, @bhimrajyadav) for the design and maintenance of SimpleProfiler and AdvancedProfiler.