[FX] make map_arg require a callable (#51907)
Summary:
This makes something like: `map_arg(lambda x: x, [Node(), Node()])` throw an error (before it would silently return `lambda x: x`)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51907
Reviewed By: jamesr66a
Differential Revision: D26323916
Pulled By: jansel
fbshipit-source-id: f56ebcf9a3af47546d75603567025163f1fb8454
