Adds Anchor tests (#2971)

* tries adding anchor tests

* fixes lint

* tries fixing

* tries one more time

* fixes the test

test/test_models_detection_anchor_utils.py

+import torch
+import unittest
+from torchvision.models.detection.anchor_utils import AnchorGenerator
+from torchvision.models.detection.image_list import ImageList
+
+
+class Tester(unittest.TestCase):
+    def test_incorrect_anchors(self):
+        incorrect_sizes = ((2, 4, 8), (32, 8), )
+        incorrect_aspects = (0.5, 1.0)
+        anc = AnchorGenerator(incorrect_sizes, incorrect_aspects)
+        image1 = torch.randn(3, 800, 800)
+        image_list = ImageList(image1, [(800, 800)])
+        feature_maps = [torch.randn(1, 50)]
+        self.assertRaises(ValueError, anc, image_list, feature_maps)