Accuracy metric for preds at half precision is zero with pl=1.0.8
🐛 Bug

The accuracy metric is wrong if preds are given with half precision. See example.
To Reproduce

import torch
from pytorch_lightning.metrics import Accuracy

acc = Accuracy(threshold=0.5)
target = torch.Tensor([1, 1, 0, 0])
preds = torch.Tensor([0.7, 0.4, 0.8, 0.4])

print(acc(preds, target))  -> 0.5
print(acc(preds.half(), target))  -> 0.0
Expected behavior

The accuracy metric should not fail silently. Either an Error needs to be raised when preds are half precision or it should work correctly.
Environment

PyTorch Version (e.g., 1.0): 1.7.0
OS (e.g., Linux): Linux
How you installed PyTorch (conda, pip, source): conda
Build command you used (if compiling from source):
Python version: 3.8
CUDA/cuDNN version: 10.2
GPU models and configuration: ...
Any other relevant information:

Additional context
This might already be fixed in master. I filed the issue regardless because I don't have time to check.