"Unsupported `ReduceOp` for distributed computing" warning when using Result without distributed
🐛 Bug

step_result.py imports pytorch_lightning.metrics.converters
converters.py raises the following warning if torch.distributed.ReduceOp cannot be imported:

rank_zero_warn('Unsupported ReduceOp for distributed computing.')

I don't use and don't want to use distributed training, but this warning is printed to stdout non-stop at one warning per second rate.
To Reproduce
Use Result object without a distributed package available.

Expected behavior

This warning should be printed once at most.
If I don't use distributed, I don't need to see this warning at all.
Environment

CUDA:
- GPU:
- GeForce RTX 2080
- available:         True
- version:           10.2
Packages:
- numpy:             1.18.1
- pyTorch_debug:     False
- pyTorch_version:   1.6.0
- pytorch-lightning: 0.9.0
- tensorboard:       2.2.0
- tqdm:              4.47.0
System:
- OS:                Windows
- architecture:
- 64bit
- WindowsPE
- processor:         Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
- python:            3.7.1
- version:           10.0.19041

Additional context