Make code for image downloading from image urls cacheable (#4218)

* Make code for image downloading from image urls cacheable

* Minor improvement in RedCaps card

* Minor fixes in formatting

Commit from https://github.com/huggingface/datasets/commit/6a201a6f8fb8837f37925e38c4cc69e92155120f

README.md

@@ -77,13 +77,16 @@ from datasets import load_dataset
 from datasets.utils.file_utils import get_datasets_user_agent
 
 
+USER_AGENT = get_datasets_user_agent()
+
+
 def fetch_single_image(image_url, timeout=None, retries=0):
     for _ in range(retries + 1):
         try:
             request = urllib.request.Request(
                 image_url,
                 data=None,
-                headers={"user-agent": get_datasets_user_agent()},
+                headers={"user-agent": USER_AGENT},
             )
             with urllib.request.urlopen(request, timeout=timeout) as req:
                 image = PIL.Image.open(io.BytesIO(req.read()))