Fix memory leak when using tf.layers

Uses `weakref` so that PER_GRAPH_LAYER_NAME_UIDS doesn't prevent Graphs from being garbage collected.

Fixes tensorflow#11273

tensorflow/python/layers/base.py

@@ -27,6 +27,7 @@
 import copy
 import functools
 import re
+import weakref
 
 from six.moves import xrange  # pylint: disable=redefined-builtin
 import numpy as np
@@ -694,6 +695,6 @@ def _unique_layer_name(name):
   ```
   """
   graph = ops.get_default_graph()
-  layer_name_uids = PER_GRAPH_LAYER_NAME_UIDS[graph]
+  layer_name_uids = PER_GRAPH_LAYER_NAME_UIDS[weakref.ref(graph)]
   layer_name_uids[name] += 1
   return name + '_' + str(layer_name_uids[name])