DDP_SPAWN + Double Precision gives pickle error
🐛 Bug

Pickle error with double precision + DDP-spawn
Can't pickle <function BoringModel.training_step at 0x7f8d684573b0>: it's not the same object as __main__.BoringModel.training_step
Fixing general pickle error may also solve discussion #6851
Please reproduce using the BoringModel

To Reproduce
Use bug_report_model.py, add accelerator='ddp-spawn' and precision=64