Move convert_to_rgb to image_transforms module (#20784)

* Move convert_to_rgb to image_transforms module

* Fix tests

tests/test_image_transforms.py

@@ -37,6 +37,7 @@ if is_vision_available():
     from transformers.image_transforms import (
         center_crop,
         center_to_corners_format,
+        convert_to_rgb,
         corners_to_center_format,
         get_resize_output_image_size,
         id_to_rgb,
@@ -456,3 +457,32 @@ class ImageTransformsTester(unittest.TestCase):
         self.assertTrue(
             np.allclose(expected_image, pad(image, ((0, 2), (2, 1)), mode="reflect", data_format="channels_last"))
         )
+
+    @require_vision
+    def test_convert_to_rgb(self):
+        # Test that an RGBA image is converted to RGB
+        image = np.array([[[1, 2, 3, 4], [5, 6, 7, 8]]], dtype=np.uint8)
+        pil_image = PIL.Image.fromarray(image)
+        self.assertEqual(pil_image.mode, "RGBA")
+        self.assertEqual(pil_image.size, (2, 1))
+
+        # For the moment, numpy images are returned as is
+        rgb_image = convert_to_rgb(image)
+        self.assertEqual(rgb_image.shape, (1, 2, 4))
+        self.assertTrue(np.allclose(rgb_image, image))
+
+        # And PIL images are converted
+        rgb_image = convert_to_rgb(pil_image)
+        self.assertEqual(rgb_image.mode, "RGB")
+        self.assertEqual(rgb_image.size, (2, 1))
+        self.assertTrue(np.allclose(np.array(rgb_image), np.array([[[1, 2, 3], [5, 6, 7]]], dtype=np.uint8)))
+
+        # Test that a grayscale image is converted to RGB
+        image = np.array([[0, 255]], dtype=np.uint8)
+        pil_image = PIL.Image.fromarray(image)
+        self.assertEqual(pil_image.mode, "L")
+        self.assertEqual(pil_image.size, (2, 1))
+        rgb_image = convert_to_rgb(pil_image)
+        self.assertEqual(rgb_image.mode, "RGB")
+        self.assertEqual(rgb_image.size, (2, 1))
+        self.assertTrue(np.allclose(np.array(rgb_image), np.array([[[0, 0, 0], [255, 255, 255]]], dtype=np.uint8)))