Added dataset download support in fbcode (#3823) (#3826)

Summary:
Pull Request resolved: https://github.com/pytorch/vision/pull/3823

Uploaded FashionMNIST dataset to [manifold](https://www.internalfb.com/intern/network/manifold/?bucket=torchvision&path=tree%2Fdatasets) bucket `torchvision`. Any new dataset that needs to be added could be uploaded under `tree/datasets/<dataset_name>`.

Reviewed By: datumbox

Differential Revision: D28358470

fbshipit-source-id: 6f2282d3f1ce4b1416e962de8fb132896d4b2d76

torchvision/datasets/_utils.py

+def _download_file_from_remote_location(fpath: str) -> None:
+    pass
+
+
+def _is_remote_location_available() -> bool:
+    return False

torchvision/datasets/utils.py

@@ -17,6 +17,11 @@ import pathlib
 import torch
 from torch.utils.model_zoo import tqdm
 
+from ._utils import (
+    _download_file_from_remote_location,
+    _is_remote_location_available,
+)
+
 
 USER_AGENT = "pytorch/vision"
 
@@ -117,6 +122,9 @@ def download_url(
         print('Using downloaded and verified file: ' + fpath)
         return
 
+    if _is_remote_location_available():
+        _download_file_from_remote_location(fpath)
+    else:
         # expand redirect chain if needed
         url = _get_redirect_url(url, max_hops=max_redirect_hops)
 
@@ -137,6 +145,7 @@ def download_url(
                 _urlretrieve(url, fpath)
             else:
                 raise e
+
     # check integrity of downloaded file
     if not check_integrity(fpath, md5):
         raise RuntimeError("File not found or corrupted.")