DDP breaks LR finder
🐛 Bug
DDP breaks LR finder
To Reproduce
finder = trainer.lr_find(model)
print(finder.suggestion())

Traceback (most recent call last):
  File "./training.py", line 107, in <module>
    main(hparam_trial)
  File "./training.py", line 97, in main
    finder = trainer.lr_find(model)
  File "/opt/conda/lib/python3.6/site-packages/pytorch_lightning/trainer/lr_finder.py", line 153, in lr_find
    self.fit(model, train_dataloader=train_dataloader)
  File "/opt/conda/lib/python3.6/site-packages/pytorch_lightning/trainer/trainer.py", line 751, in fit
    mp.spawn(self.ddp_train, nprocs=self.num_processes, args=(model,))
  File "/opt/conda/lib/python3.6/site-packages/torch/multiprocessing/spawn.py", line 162, in spawn
    process.start()
  File "/opt/conda/lib/python3.6/multiprocessing/process.py", line 105, in start
    self._popen = self._Popen(self)
  File "/opt/conda/lib/python3.6/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/opt/conda/lib/python3.6/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/opt/conda/lib/python3.6/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/opt/conda/lib/python3.6/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/opt/conda/lib/python3.6/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
AttributeError: Can't pickle local object '_LRFinder._get_new_optimizer.<locals>.configure_optimizers'

At first I thought it's because configure_optimizers returns [opt], [sched] but returning opt still causes the error. Training works correctly with the same code.