Check if mask_dir is available for segmentation task in Folder data (#1277)

* Fix metadata path

* Ignore hidden directories in folder dataset

* Add check for mask_dir for segmentation tasks in Folder dataset

* Check the test split mode when no mask dir is provided

Author: samet-akcay

src/anomalib/data/folder.py

@@ -281,8 +281,13 @@ def __init__(
             seed=seed,
         )
 
-        self.normal_split_ratio = normal_split_ratio
+        if task == TaskType.SEGMENTATION and test_split_mode == TestSplitMode.FROM_DIR and mask_dir is None:
+            raise ValueError(
+                f"Segmentation task requires mask directory if test_split_mode is {test_split_mode}. "
+                f"You could set test_split_mode to {TestSplitMode.NONE} or provide a mask directory."
+            )
 
+        self.normal_split_ratio = normal_split_ratio
         transform_train = get_transforms(
             config=transform_config_train,
             image_size=image_size,