fix(train): guard checkpoint saving against save_freq=0 (#4112)

is_saving_step was the only step-frequency check without a `> 0` guard,
so save_freq=0 raised ZeroDivisionError from `step % 0` on the first
step. Route the decision through a should_save_checkpoint helper that
treats a non-positive save_freq as "save only the final checkpoint",
matching how log_freq/eval_freq handle non-positive values.

Co-authored-by: Steven Palma <imstevenpmwork@ieee.org>

tests/utils/test_train_utils.py

@@ -30,6 +30,7 @@ from lerobot.common.train_utils import (
     save_checkpoint,
     save_training_state,
     save_training_step,
+    should_save_checkpoint,
     update_last_checkpoint,
 )
 from lerobot.utils.constants import (
@@ -50,6 +51,18 @@ def test_get_step_identifier():
     assert get_step_identifier(456789, 1_000_000) == "0456789"
 
 
+def test_should_save_checkpoint():
+    # Periodic checkpoints land on multiples of save_freq.
+    assert should_save_checkpoint(10, save_freq=10, total_steps=100) is True
+    assert should_save_checkpoint(5, save_freq=10, total_steps=100) is False
+    # The final step always saves, even when it is not a multiple of save_freq.
+    assert should_save_checkpoint(100, save_freq=30, total_steps=100) is True
+    # save_freq <= 0 disables periodic saving without raising ZeroDivisionError.
+    assert should_save_checkpoint(1, save_freq=0, total_steps=100) is False
+    assert should_save_checkpoint(100, save_freq=0, total_steps=100) is True
+    assert should_save_checkpoint(1, save_freq=-1, total_steps=100) is False
+
+
 def test_get_step_checkpoint_dir():
     output_dir = Path("/checkpoints")
     step_dir = get_step_checkpoint_dir(output_dir, 1000, 5)