fix: correct LR warmdown step range

scripts/base_train.py

@@ -238,15 +238,28 @@ x, y, dataloader_state_dict = next(train_loader) # kick off load of the very fir
 
 # Learning rate scheduler
 def get_lr_multiplier(it):
-    warmup_iters = round(args.warmup_ratio * num_iterations)
-    warmdown_iters = round(args.warmdown_ratio * num_iterations)
-    if it < warmup_iters:
+    # Note: optimizer steps run for it in [0, num_iterations-1]
+    warmup_iters = int(round(args.warmup_ratio * num_iterations))
+    warmdown_iters = int(round(args.warmdown_ratio * num_iterations))
+
+    # Warmup (avoid division by zero when warmup_iters == 0)
+    if warmup_iters > 0 and it < warmup_iters:
         return (it + 1) / warmup_iters
-    elif it <= num_iterations - warmdown_iters:
-        return 1.0
-    else:
-        progress = (num_iterations - it) / warmdown_iters
-        return progress * 1.0 + (1 - progress) * args.final_lr_frac
+
+    # Warmdown should cover the last `warmdown_iters` optimizer steps:
+    # it in [num_iterations - warmdown_iters, num_iterations - 1]
+    if warmdown_iters > 0:
+        warmdown_start = num_iterations - warmdown_iters
+        # Ensure warmdown doesn't start before warmup ends (prevents overlap weirdness)
+        warmdown_start = max(warmdown_start, warmup_iters)
+
+        if it >= warmdown_start:
+            # progress: 1.0 at warmdown_start, 0.0 at last optimizer step (num_iterations - 1)
+            span = max(1, (num_iterations - 1) - warmdown_start)  # denom >= 1
+            progress = (num_iterations - 1 - it) / span
+            return progress * 1.0 + (1.0 - progress) * args.final_lr_frac
+
+    return 1.0
 
 # Momentum scheduler for Muon optimizer
 def get_muon_momentum(it):