Add worker_fn argument to multiworker function (#12177)

* add worker_fn argument to multiworker function

* fix pylin

python/mxnet/gluon/data/dataloader.py

@@ -183,7 +183,8 @@ def fetcher_loop(data_queue, data_buffer, pin_memory=False):
 
 class _MultiWorkerIter(object):
     """Interal multi-worker iterator for DataLoader."""
-    def __init__(self, num_workers, dataset, batchify_fn, batch_sampler, pin_memory=False):
+    def __init__(self, num_workers, dataset, batchify_fn, batch_sampler, pin_memory=False,
+                 worker_fn=worker_loop):
         assert num_workers > 0, "_MultiWorkerIter is not for {} workers".format(num_workers)
         self._num_workers = num_workers
         self._dataset = dataset
@@ -200,7 +201,7 @@ def __init__(self, num_workers, dataset, batchify_fn, batch_sampler, pin_memory=
         workers = []
         for _ in range(self._num_workers):
             worker = multiprocessing.Process(
-                target=worker_loop,
+                target=worker_fn,
                 args=(self._dataset, self._key_queue, self._data_queue, self._batchify_fn))
             worker.daemon = True
             worker.start()