accuracy metric dosen't support windows
🐛 Bug
Pytorch Metric.Accuracy uses ReduceOp from 'torch.distribution' but torch.distributrion doesn't support windows

https://github.com/pytorch/pytorch/blob/cf8a9b50cacb1702f5855859c657a5358976437b/torch/distributed/__init__.py#L10 : torch.distributed is available on Linux and MacOS.

To Reproduce
Use Metric.Accuracy in Windows environment

Code sample

I use code sample from https://github.com/PyTorchLightning/pytorch-lightning/issues/2355

Expected behavior
add check OS in metric.accuracy and use condition for import different module
  try:
    return platform.linux_distribution()
  except:
    return "N/A"

or warning to windows user, they can't use metric.accuracy
Environment
* CUDA:
        - GPU:
                - GeForce RTX 2080 Ti
                - GeForce GTX 1080 Ti
        - available:         True
        - version:           10.1
* Packages:
        - numpy:             1.18.1
        - pyTorch_debug:     False
        - pyTorch_version:   1.5.1
        - pytorch-lightning: 0.8.1
        - tensorboard:       2.2.1
        - tqdm:              4.46.0
* System:
        - OS:                Windows
        - architecture:
                - 64bit
                - WindowsPE
        - processor:         Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
        - python:            3.6.10
        - version:           10.0.18362

Additional context
Traceback (most recent call last):
  File "test.py", line 11, in <module>
    from pytorch_lightning.metrics.functional import accuracy
  File "C:\Users\dcho\Anaconda3\envs\torch_py36\lib\site-packages\pytorch_lightning\metrics\__init__.py", line 1, in <module>
    from pytorch_lightning.metrics.converters import numpy_metric, tensor_metric
  File "C:\Users\dcho\Anaconda3\envs\torch_py36\lib\site-packages\pytorch_lightning\metrics\converters.py", line 220, in <module>
    reduce_op: Optional[torch.distributed.ReduceOp] = None,
AttributeError: module 'torch.distributed' has no attribute 'ReduceOp'


Always thanks for developing & maintaining the cool framework