Rename Saver used in init_fn

Bug:
We use two different Saver objects during training, one to initialize from a
checkpoint and another to save and restore from checkpoints during training.
Both these Savers were bound to same variable name causing init_fn function
to use the latter instead of the former when called.

object_detection/trainer.py

@@ -213,11 +213,12 @@ def train(create_tensor_dict_fn, create_model_fn, train_config, master, task,
     if train_config.fine_tune_checkpoint:
       var_map = detection_model.restore_map(
           from_detection_checkpoint=train_config.from_detection_checkpoint)
-      var_map = variables_helper.get_variables_available_in_checkpoint(
-          var_map, train_config.fine_tune_checkpoint)
-      saver = tf.train.Saver(var_map)
+      available_var_map = (variables_helper.
+                           get_variables_available_in_checkpoint(
+                               var_map, train_config.fine_tune_checkpoint))
+      init_saver = tf.train.Saver(available_var_map)
       def initializer_fn(sess):
-        saver.restore(sess, train_config.fine_tune_checkpoint)
+        init_saver.restore(sess, train_config.fine_tune_checkpoint)
       init_fn = initializer_fn
 
     with tf.device(deploy_config.optimizer_device()):