fix(train): make NaN fast-fail check explicit

train.py

@@ -9,6 +9,7 @@ os.environ["PYTORCH_ALLOC_CONF"] = "expandable_segments:True"
 os.environ["HF_HUB_DISABLE_PROGRESS_BARS"] = "1"
 
 import gc
+import math
 import time
 from dataclasses import dataclass, asdict
 
@@ -566,7 +567,7 @@ while True:
     train_loss_f = train_loss.item()
 
     # Fast fail: abort if loss is exploding or NaN
-    if not train_loss_f <= 100:
+    if math.isnan(train_loss_f) or train_loss_f > 100:
         print("FAIL")
         exit(1)