fix to work linear/cosine scheduler closes #1651 ref #1393

Author: kohya-ss

library/train_util.py

@@ -4496,6 +4496,15 @@ def wrap_check_needless_num_warmup_steps(return_vals):
             **lr_scheduler_kwargs,
         )
 
+    # these schedulers do not require `num_decay_steps`
+    if name == SchedulerType.LINEAR or name == SchedulerType.COSINE:
+        return schedule_func(
+            optimizer,
+            num_warmup_steps=num_warmup_steps,
+            num_training_steps=num_training_steps,
+            **lr_scheduler_kwargs,
+        )
+
     # All other schedulers require `num_decay_steps`
     if num_decay_steps is None:
         raise ValueError(f"{name} requires `num_decay_steps`, please provide that argument.")