Make transformers-cli cross-platform (#4131)

* make transformers-cli cross-platform

Using "scripts" is a useful option in setup.py particularly when you want to get access to non-python scripts. However, in this case we want to have an entry point into some of our own Python scripts. To do this in a concise, cross-platfom way, we can use entry_points.console_scripts. This change is necessary to provide the CLI on different platforms, which "scripts" does not ensure. Usage remains the same, but the "transformers-cli" script has to be moved (be part of the library) and renamed (underscore + extension)

* make style & quality

src/transformers/commands/transformers_cli.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python
+from argparse import ArgumentParser
+
+from transformers.commands.convert import ConvertCommand
+from transformers.commands.download import DownloadCommand
+from transformers.commands.env import EnvironmentCommand
+from transformers.commands.run import RunCommand
+from transformers.commands.serving import ServeCommand
+from transformers.commands.user import UserCommands
+
+
+def main():
+    parser = ArgumentParser("Transformers CLI tool", usage="transformers-cli <command> [<args>]")
+    commands_parser = parser.add_subparsers(help="transformers-cli command helpers")
+
+    # Register commands
+    ConvertCommand.register_subcommand(commands_parser)
+    DownloadCommand.register_subcommand(commands_parser)
+    EnvironmentCommand.register_subcommand(commands_parser)
+    RunCommand.register_subcommand(commands_parser)
+    ServeCommand.register_subcommand(commands_parser)
+    UserCommands.register_subcommand(commands_parser)
+
+    # Let's go
+    args = parser.parse_args()
+
+    if not hasattr(args, "func"):
+        parser.print_help()
+        exit(1)
+
+    # Run
+    service = args.func(args)
+    service.run()
+
+
+if __name__ == "__main__":
+    main()