don't assume default device in towercontext (#295)

tensorpack/tfutils/tower.py

@@ -29,8 +29,6 @@ class TowerContext(object):
                 'replicated' mode. Defaults to be tower_name.
         """
         self._name = tower_name
-        if device is None:
-            device = '/gpu:0' if tf.test.is_gpu_available() else '/cpu:0'
         self._device = device
 
         if is_training is None:
@@ -129,7 +127,8 @@ class TowerContext(object):
                             tf.get_variable_scope(), reuse=True))
                 # if not training, should handle vs outside (TODO not good)
                 self._ctxs.append(tf.name_scope(self._name))
-        self._ctxs.append(tf.device(self._device))
+        if self._device is not None:
+            self._ctxs.append(tf.device(self._device))
         for c in self._ctxs:
             c.__enter__()