Missing `return` in `__call__` of transforms: `SaveImage`, `NiftiSaver` and `PNGSaver`

**To Reproduce**
Steps to reproduce the behavior:

```
from monai.transforms import SaveImage, Compose
saver = Compose([SaveImage(output_dir="./output", output_ext=".png", output_postfix="seg")])
img = torch.randn([3, 32, 32])
output = saver(img)
print(output)
```