        module_or_grads_list: Either a network definition (module) being run in multi-gpu/distributed mode, or an iterable of gradients to be reduced.  If a module is passed in, the Reducer constructor will sync the parameters across processes (broadcasting from rank 0) to make sure they're all initialized with the same values.  If a list of gradients (that came from some module) is passed in, the user is responsible for manually syncing that module's parameters at the beginning of training.
