Fix ModuleNotFoundError: No module named 'skimage' when importing tor…

…cheval (#205)

Summary:
Prevent ModuleNotFoundError when skimage is not installed

Pull Request resolved: #205

Test Plan:
None

Fixes #204

Reviewed By: diego-urgell

Differential Revision: D62450136

Pulled By: bobakfb

fbshipit-source-id: cdc4507019d000f2352f7cd5ba25d71d12ab749c

image-requirements.txt

@@ -1 +1,2 @@
 torchvision
+skimage

torcheval/metrics/image/ssim.py

@@ -9,18 +9,32 @@
 # pyre-ignore-all-errors[16]: Undefined attribute of metric states.
 
 import warnings
+from importlib.util import find_spec
 from typing import Iterable, Optional, TypeVar
 
 import torch
 
-from skimage.metrics import structural_similarity
+if find_spec("skimage") is not None:
+    from skimage.metrics import structural_similarity
+
+    _SKIMAGE_AVAILABLE = True
+else:
+    _SKIMAGE_AVAILABLE = False
+
 
 from torcheval.metrics.metric import Metric
 
 
 TStructuralSimilarity = TypeVar("TStructuralSimilarity")
 
 
+def _validate_torchvision_available() -> None:
+    if not _SKIMAGE_AVAILABLE:
+        raise RuntimeError(
+            "You must have skimage installed to use SSIM, please install torcheval[image]"
+        )
+
+
 class StructuralSimilarity(Metric[torch.Tensor]):
     """
     Compute the structural similarity index (SSIM) between two sets of images.