fix Caltech datasets (#4556)

Co-authored-by: Prabhat Roy <prabhatroy@fb.com>

torchvision/datasets/caltech.py

@@ -133,13 +133,11 @@ class Caltech101(VisionDataset):
         download_and_extract_archive(
             "http://www.vision.caltech.edu/Image_Datasets/Caltech101/101_ObjectCategories.tar.gz",
             self.root,
-            filename="101_ObjectCategories.tar.gz",
             md5="b224c7392d521a49829488ab0f1120d9",
         )
         download_and_extract_archive(
             "http://www.vision.caltech.edu/Image_Datasets/Caltech101/Annotations.tar",
             self.root,
-            filename="101_Annotations.tar",
             md5="6f83eeb1f24d99cab4eb377263132c91",
         )
 
@@ -184,7 +182,13 @@ class Caltech256(VisionDataset):
         self.index: List[int] = []
         self.y = []
         for (i, c) in enumerate(self.categories):
-            n = len(os.listdir(os.path.join(self.root, "256_ObjectCategories", c)))
+            n = len(
+                [
+                    item
+                    for item in os.listdir(os.path.join(self.root, "256_ObjectCategories", c))
+                    if item.endswith(".jpg")
+                ]
+            )
             self.index.extend(range(1, n + 1))
             self.y.extend(n * [i])