`UserWarning: Your validation_step returned None` is issued when using DDP

[dpieczynski]

🐛 Bug

When using DDP with the validation_step function that doesn't return anything the following warning is printed (Lightning 1.2.0):

pytorch_lightning/utilities/distributed.py:50: UserWarning: Your validation_step returned None. Did you forget to return an output?

This doesn't seem right, as documentation, as far as I'm aware, never mentions a need for doing so.

To Reproduce

Run .fit(...) on a Trainer created with accelerator='ddp'.

Environment

• PyTorch Version (e.g., 1.0): 1.7.1
• OS (e.g., Linux): Linux
• How you installed PyTorch (conda, pip, source): pip
• Python version: 3.8.6