add initial pass at migrating AG_NEWS to datapipes.

Author: erip

torchtext/datasets/ag_news.py

@@ -1,12 +1,12 @@
-from torchtext.utils import (
-    download_from_url,
-)
+from torchtext._internal.module_utils import is_module_available
+
+if is_module_available("torchdata"):
+    from torchdata.datapipes.iter import FileOpener, HttpReader, IterableWrapper
+
 from torchtext.data.datasets_utils import (
-    _RawTextIterableDataset,
     _wrap_split_argument,
     _add_docstring_header,
     _create_dataset_directory,
-    _create_data_from_csv,
 )
 import os
 
@@ -30,11 +30,15 @@
 
 @_add_docstring_header(num_lines=NUM_LINES, num_classes=4)
 @_create_dataset_directory(dataset_name=DATASET_NAME)
-@_wrap_split_argument(('train', 'test'))
+@_wrap_split_argument(("train", "test"))
 def AG_NEWS(root, split):
-    path = download_from_url(URL[split], root=root,
-                             path=os.path.join(root, split + ".csv"),
-                             hash_value=MD5[split],
-                             hash_type='md5')
-    return _RawTextIterableDataset(DATASET_NAME, NUM_LINES[split],
-                                   _create_data_from_csv(path))
+    url_dp = IterableWrapper([URL[split]])
+    http_dp = HttpReader(url_dp)
+    cache_dp = http_dp.on_disk_cache(
+        filepath_fn=lambda x: os.path.join(root, split + ".csv"),
+        hash_dict={os.path.join(root, split + ".csv"): MD5[split]},
+        hash_type="md5"
+    ).end_caching(mode="w", same_filepath_fn=True)
+
+    cache_dp = FileOpener(cache_dp, mode="r")
+    return cache_dp.parse_csv().map(fn=lambda t: (int(t[0]), t[1]))