use tf.AUTO_REUSE, add reuse arg to under_variable_scope and fix (#607)

* use tf.AUTO_REUSE, and fix plasma.putdata

* revert auto-reuse and add tf.AUTO_REUSE just as an option

* revert

tensorpack/dataflow/parallel.py

@@ -421,5 +421,6 @@ class PlasmaGetData(ProxyDataFlow):
 try:
     import pyarrow.plasma as plasma
 except ImportError:
+    from ..utils.develop import create_dummy_class
     PlasmaPutData = create_dummy_class('PlasmaPutData', 'pyarrow')   # noqa
     PlasmaGetData = create_dummy_class('PlasmaGetData', 'pyarrow')   # noqa