Error on example model
🐛 Bug
I receive on every GAN model that I try to execute.
RuntimeError: Expected to have finished reduction in the prior iteration before starting a new one. This error indicates that your module has parameters that were not used in producing loss. You can enable unused parameter detection by passing the keyword argument `find_unused_parameters=True` to `torch.nn.parallel.DistributedDataParallel`, and by 
making sure all `forward` function outputs participate in calculating loss. 
If you already have done the above, then the distributed data parallel module wasn't able to locate the output tensors in the return value of your module's `forward` function. Please include the loss function and the structure of the return value of `forward` of your module when reporting this issue (e.g. list, dict, iterable).

I tried compete reinstall of pytorch in different conda environment, tried both 1.8.0 and 1.9.0.dev
I get error on both GAN example and my custom model.
My custom model has only 1 parameter (each of the networks), that I multiply with input, but it gives error too.
To Reproduce
GAN model from examples
Expected behavior

Environment
PyTorch 1.8.0 (or latest dev)
PyTorch-Lightning 1.2.3
Additional context
I run on 2 Nvidia V100 gpus in ddp mode