use enums in prototype datasets for demux (#5189)

* use enums in prototype datasets for demux

* use enum for category generation

* revert enum usage for single use constants

torchvision/prototype/datasets/_builtin/dtd.py

+import enum
 import io
 import pathlib
 from typing import Any, Callable, Dict, List, Optional, Tuple
@@ -30,6 +31,12 @@ from torchvision.prototype.datasets.utils._internal import (
 from torchvision.prototype.features import Label
 
 
+class DTDDemux(enum.IntEnum):
+    SPLIT = 0
+    JOINT_CATEGORIES = 1
+    IMAGES = 2
+
+
 class DTD(Dataset):
     def _make_info(self) -> DatasetInfo:
         return DatasetInfo(
@@ -54,11 +61,11 @@ class DTD(Dataset):
         path = pathlib.Path(data[0])
         if path.parent.name == "labels":
             if path.name == "labels_joint_anno.txt":
-                return 1
+                return DTDDemux.JOINT_CATEGORIES
 
-            return 0
+            return DTDDemux.SPLIT
         elif path.parents[1].name == "images":
-            return 2
+            return DTDDemux.IMAGES
         else:
             return None
 
@@ -122,7 +129,7 @@ class DTD(Dataset):
         return Mapper(dp, self._collate_and_decode_sample, fn_kwargs=dict(decoder=decoder))
 
     def _filter_images(self, data: Tuple[str, Any]) -> bool:
-        return self._classify_archive(data) == 2
+        return self._classify_archive(data) == DTDDemux.IMAGES
 
     def _generate_categories(self, root: pathlib.Path) -> List[str]:
         dp = self.resources(self.default_config)[0].load(pathlib.Path(root) / self.name)

torchvision/prototype/datasets/_builtin/oxford_iiit_pet.py

+import enum
 import functools
 import io
 import pathlib
@@ -24,6 +25,11 @@ from torchvision.prototype.datasets.utils._internal import (
 from torchvision.prototype.features import Label
 
 
+class OxfordIITPetDemux(enum.IntEnum):
+    SPLIT_AND_CLASSIFICATION = 0
+    SEGMENTATIONS = 1
+
+
 class OxfordIITPet(Dataset):
     def _make_info(self) -> DatasetInfo:
         return DatasetInfo(
@@ -51,8 +57,8 @@ class OxfordIITPet(Dataset):
 
     def _classify_anns(self, data: Tuple[str, Any]) -> Optional[int]:
         return {
-            "annotations": 0,
-            "trimaps": 1,
+            "annotations": OxfordIITPetDemux.SPLIT_AND_CLASSIFICATION,
+            "trimaps": OxfordIITPetDemux.SEGMENTATIONS,
         }.get(pathlib.Path(data[0]).parent.name)
 
     def _filter_images(self, data: Tuple[str, Any]) -> bool:
@@ -135,7 +141,7 @@ class OxfordIITPet(Dataset):
         return Mapper(dp, functools.partial(self._collate_and_decode_sample, decoder=decoder))
 
     def _filter_split_and_classification_anns(self, data: Tuple[str, Any]) -> bool:
-        return self._classify_anns(data) == 0
+        return self._classify_anns(data) == OxfordIITPetDemux.SPLIT_AND_CLASSIFICATION
 
     def _generate_categories(self, root: pathlib.Path) -> List[str]:
         config = self.default_config