Change steps_per_loop to `None` by default.

PiperOrigin-RevId: 321855387

official/vision/image_classification/resnet/common.py

@@ -255,7 +255,7 @@ def define_keras_flags(
       name='tpu', default='', help='TPU address to connect to.')
   flags.DEFINE_integer(
       name='steps_per_loop',
-      default=500,
+      default=None,
       help='Number of steps per training loop. Only training step happens '
       'inside the loop. Callbacks will not be called inside. Will be capped at '
       'steps per epoch.')

official/vision/image_classification/resnet/resnet_ctl_imagenet_main.py

@@ -125,7 +125,7 @@ def run(flags_obj):
 
   per_epoch_steps, train_epochs, eval_steps = get_num_train_iterations(
       flags_obj)
-  if not flags_obj.steps_per_loop:
+  if flags_obj.steps_per_loop is None:
     steps_per_loop = per_epoch_steps
   elif flags_obj.steps_per_loop > per_epoch_steps:
     steps_per_loop = per_epoch_steps