[BUG] s3_client not defined when downloading Open Images dataset
**OS version**: macOS 10.14.6
**Python version**: 3.8
**FiftyOne version**: 0.9.1

When trying to download + load the Open Images dataset:

```py
import fiftyone.zoo as foz

dataset = foz.load_zoo_dataset(
    "open-images-v6",
    split="validation",
    label_types=["detections"],
    max_samples=10,
)
```

I encounter the following error:

```
-> 1275     _download_specific_images(valid_ids, split, dataset_dir, num_workers)
   1276 
   1277     if "segmentations" in label_types:
/opt/anaconda3/lib/python3.8/site-packages/fiftyone/utils/openimages.py in _download_specific_images(valid_ids, split, dataset_dir, num_workers, ext)
   1565     with fou.ProgressBar(total=len(inputs)) as pb:
   1566         with multiprocessing.Pool(num_workers) as pool:
-> 1567             for _ in pool.imap_unordered(_do_s3_download, inputs):
   1568                 pb.update()
   1569 
/opt/anaconda3/lib/python3.8/multiprocessing/pool.py in next(self, timeout)
    866         if success:
    867             return value
--> 868         raise value
    869 
    870     __next__ = next                    # XXX
NameError: name 's3_client' is not defined
```
diff --git a/fiftyone/utils/openimages.py b/fiftyone/utils/openimages.py
index 3272abbce..af6ba93ab 100644
--- a/fiftyone/utils/openimages.py
+++ b/fiftyone/utils/openimages.py
@@ -1559,19 +1559,14 @@ def _download_specific_images(
             len(inputs),
         )
 
-    global s3_client
-    s3_client = None
-
     with fou.ProgressBar(total=len(inputs)) as pb:
-        with multiprocessing.Pool(num_workers) as pool:
+        with multiprocessing.Pool(num_workers, _initialize_worker) as pool:
             for _ in pool.imap_unordered(_do_s3_download, inputs):
                 pb.update()
 
 
-def _initialize_pool():
+def _initialize_worker():
     global s3_client
-    if s3_client:
-        return
     s3_client = boto3.client(
         "s3",
         config=botocore.config.Config(signature_version=botocore.UNSIGNED),
@@ -1579,7 +1574,6 @@ def _initialize_pool():
 
 
 def _do_s3_download(args):
-    _initialize_pool()
     filepath, filepath_download = args
     s3_client.download_file(_BUCKET_NAME, filepath_download, filepath)
 
['fiftyone/utils/openimages.py']