fix: return inf instead of crashing when evaluate_bpb has zero total_bytes

Edge case: all tokens are special tokens or ignored

Return infinity to indicate no meaningful bytes were processed

nanochat/loss_eval.py

@@ -59,5 +59,7 @@ def evaluate_bpb(model, batches, steps, token_bytes):
     # move both to cpu, calculate bpb and return
     total_nats = total_nats.item()
     total_bytes = total_bytes.item()
+    if total_bytes == 0:
+        return float('inf')
     bpb = total_nats / (math.log(2) * total_bytes)
     return bpb