make save_checkpoints_secs work again (#521)

The functionality was broken during the adoption of TPU trainer_lib.py
instead of the original trainer_utils.py.
Currently, the default is to save checkpoints each 2000 steps,
while in previous T2T versions the default was each 10 minutes.

tensor2tensor/bin/t2t_trainer.py

@@ -108,14 +108,17 @@ def create_experiment_fn():
 
 
 def create_run_config(hp):
+  save_ckpt_steps = max(FLAGS.iterations_per_loop, FLAGS.local_eval_frequency)
+  if FLAGS.save_checkpoints_secs:
+    save_ckpt_steps = None
   return trainer_lib.create_run_config(
       model_dir=os.path.expanduser(FLAGS.output_dir),
       master=FLAGS.master,
       iterations_per_loop=FLAGS.iterations_per_loop,
       num_shards=FLAGS.tpu_num_shards,
       log_device_placement=FLAGS.log_device_placement,
-      save_checkpoints_steps=max(FLAGS.iterations_per_loop,
-                                 FLAGS.local_eval_frequency),
+      save_checkpoints_steps=save_ckpt_steps,
+      save_checkpoints_secs=FLAGS.save_checkpoints_secs,
       keep_checkpoint_max=FLAGS.keep_checkpoint_max,
       keep_checkpoint_every_n_hours=FLAGS.keep_checkpoint_every_n_hours,
       num_gpus=FLAGS.worker_gpu,

tensor2tensor/utils/flags.py

@@ -81,8 +81,8 @@
                      "The default value 10,000 hours effectively disables it.")
 flags.DEFINE_integer("save_checkpoints_secs", 0,
                      "Save checkpoints every this many seconds. "
-                     "Default=0 means let tensorflow.contrib.learn.python.learn"
-                     " decide, which is currently set to 600 = 10 minutes.")
+                     "Default=0 means save checkpoints each x steps where x "
+                     "depends on iterations_per_loop and local_eval_frequency.")
 flags.DEFINE_bool("log_device_placement", False,
                   "Whether to log device placement.")
 

tensor2tensor/utils/trainer_lib.py

@@ -87,6 +87,7 @@ def create_run_config(master="",
                       num_shards=8,
                       log_device_placement=False,
                       save_checkpoints_steps=1000,
+                      save_checkpoints_secs=0,
                       keep_checkpoint_max=20,
                       keep_checkpoint_every_n_hours=10000,
                       num_gpus=1,
@@ -121,6 +122,7 @@ def create_run_config(master="",
       "session_config": session_config,
       "save_summary_steps": 100,
       "save_checkpoints_steps": save_checkpoints_steps,
+      "save_checkpoints_secs": save_checkpoints_secs,
       "keep_checkpoint_max": keep_checkpoint_max,
       "keep_checkpoint_every_n_hours": keep_checkpoint_every_n_hours,
       "tf_random_seed": random_seed,