Make the dataset streamable

This way people can do

```python
from datasets import load_dataset

ds = load_dataset("csebuetnlp/xlsum", streaming=True)
for example in ds["train"]:
...
```
without downloading the full dataset

xlsum.py

@@ -41,7 +41,7 @@ _HOMEPAGE = "https://github.com/csebuetnlp/xl-sum"
 
 _LICENSE = "Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (CC BY-NC-SA 4.0)"
 
-_URL = "https://huggingface.co/datasets/csebuetnlp/xlsum/resolve/main/data/{}_XLSum_v{}.tar.bz2"
+_DATA_PATH = "data/{}_XLSum_v{}.tar.bz2"  # relative path to the data
 
 _LANGUAGES = [
     "oromo",
@@ -125,39 +125,45 @@ class Xlsum(datasets.GeneratorBasedBuilder):
     def _split_generators(self, dl_manager):
         """Returns SplitGenerators."""
         lang = str(self.config.name)
-        url = _URL.format(lang, self.VERSION.version_str[:-2])
+        data_path = _DATA_PATH.format(lang, self.VERSION.version_str[:-2])
 
-        data_dir = dl_manager.download_and_extract(url)
+        # we download and use dl_manager.iter_archive to be able to load the dataset in streaming mode
+        archive = dl_manager.download(data_path)
         return [
             datasets.SplitGenerator(
                 name=datasets.Split.TRAIN,
                 gen_kwargs={
-                    "filepath": os.path.join(data_dir, lang + "_train.jsonl"),
+                    "filepath": lang + "_train.jsonl,
+                    "files": dl_manager.iter_archive(archive),
                 },
             ),
             datasets.SplitGenerator(
                 name=datasets.Split.TEST,
                 gen_kwargs={
-                    "filepath": os.path.join(data_dir, lang + "_test.jsonl"),
+                    "filepath": lang + "_test.jsonl",
+                    "files": dl_manager.iter_archive(archive),
                 },
             ),
             datasets.SplitGenerator(
                 name=datasets.Split.VALIDATION,
                 gen_kwargs={
-                    "filepath": os.path.join(data_dir, lang + "_val.jsonl"),
+                    "filepath": lang + "_val.jsonl",
+                    "files": dl_manager.iter_archive(archive),
                 },
             ),
         ]
 
-    def _generate_examples(self, filepath):
+    def _generate_examples(self, filepath, files):
         """Yields examples as (key, example) tuples."""
-        with open(filepath, encoding="utf-8") as f:
-            for idx_, row in enumerate(f):
-                data = json.loads(row)
-                yield idx_, {
-                    "id": data["id"],
-                    "url": data["url"],
-                    "title": data["title"],
-                    "summary": data["summary"],
-                    "text": data["text"],
-                }
+        for path, f in files:
+            if path == filepath:
+                for idx_, row in enumerate(f):
+                    data = json.loads(row)
+                    yield idx_, {
+                        "id": data["id"],
+                        "url": data["url"],
+                        "title": data["title"],
+                        "summary": data["summary"],
+                        "text": data["text"],
+                    }
+                break