Refactor download (#4384)

* Refactor download

* Fix textvqa dataset

* Move download module one level up

* Fix test mock fsspec

* Fix docs

* Fix import of DownloadConfig

* Refactor mock_download_manager

* Refactor DownloadConfig

* Fix docs

* Add sections to docs of builder classes

* Use relative imports

* Fix path

* Fix import

Commit from https://github.com/huggingface/datasets/commit/82258b47dcf6e12a9b9f2aec68ea18fb3fd3d9fc

textvqa.py

@@ -93,7 +93,7 @@ class Textvqa(datasets.GeneratorBasedBuilder):
             citation=_CITATION,
         )
 
-    def _split_generators(self, dl_manager: datasets.utils.DownloadManager):
+    def _split_generators(self, dl_manager):
         downloaded_files = dl_manager.download_and_extract(_URLS)
         return [
             datasets.SplitGenerator(