[1.x, BUGFIX] WAR the dataloader issue with forked processes holding stale references

python/mxnet/gluon/data/dataloader.py

@@ -651,6 +651,10 @@ def __init__(self, dataset, batch_size=None, shuffle=False, sampler=None,
         self._num_workers = num_workers if num_workers >= 0 else 0
         self._worker_pool = None
         self._prefetch = max(0, int(prefetch) if prefetch is not None else 2 * self._num_workers)
+        nd.waitall()
+        import gc
+        gc.collect()
+        nd.waitall()
         if self._num_workers > 0:
             if self._thread_pool:
                 self._worker_pool = ThreadPool(self._num_workers,