Added download command through the cli.

It allows to predownload models and tokenizers.

transformers-cli

 #!/usr/bin/env python
 from argparse import ArgumentParser
 
+from transformers.commands.download import DownloadCommand
 from transformers.commands.serving import ServeCommand
 from transformers.commands.user import UserCommands
 from transformers.commands.train import TrainCommand
@@ -11,10 +12,11 @@ if __name__ == '__main__':
     commands_parser = parser.add_subparsers(help='transformers-cli command helpers')
 
     # Register commands
+    ConvertCommand.register_subcommand(commands_parser)
+    DownloadCommand.register_subcommand(commands_parser)
     ServeCommand.register_subcommand(commands_parser)
     UserCommands.register_subcommand(commands_parser)
     TrainCommand.register_subcommand(commands_parser)
-    ConvertCommand.register_subcommand(commands_parser)
 
     # Let's go
     args = parser.parse_args()

transformers/commands/download.py

+from argparse import ArgumentParser
+
+from transformers.commands import BaseTransformersCLICommand
+
+
+def download_command_factory(args):
+    return DownloadCommand(args.model, args.cache_dir, args.force)
+
+
+class DownloadCommand(BaseTransformersCLICommand):
+
+    @staticmethod
+    def register_subcommand(parser: ArgumentParser):
+        download_parser = parser.add_parser('download')
+        download_parser.add_argument('--cache-dir', type=str, default=None, help='Path to location to store the models')
+        download_parser.add_argument('--force',  action='store_true', help='Force the model to be download even if already in cache-dir')
+        download_parser.add_argument('model', type=str, help='Name of the model to download')
+        download_parser.set_defaults(func=download_command_factory)
+
+    def __init__(self, model: str, cache: str, force: bool):
+        self._model = model
+        self._cache = cache
+        self._force = force
+
+    def run(self):
+        from transformers import AutoModel, AutoTokenizer
+
+        AutoModel.from_pretrained(self._model, cache_dir=self._cache, force_download=self._force)
+        AutoTokenizer.from_pretrained(self._model, cache_dir=self._cache, force_download=self._force)
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