model: silence torch parity warning (read loss before backward)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xtrain-model/tests/parity.py

@@ -134,6 +134,7 @@ logits = h @ lm_head  # [seq, vocab]
 
 loss = torch.nn.functional.cross_entropy(
     logits, torch.tensor(targets, dtype=torch.long), reduction="mean")
+loss_val = loss.item()
 loss.backward()
 
 # ---- Compare ----
@@ -147,8 +148,8 @@ def relerr(a, b):
 ref_logits = read_vec("logits.txt")
 ref_loss = read_vec("loss.txt").item()
 
-print(f"loss: rust={ref_loss:.6e} torch={loss.item():.6e} "
-      f"relerr={abs(loss.item()-ref_loss)/max(abs(ref_loss),1e-6):.2e}")
+print(f"loss: rust={ref_loss:.6e} torch={loss_val:.6e} "
+      f"relerr={abs(loss_val-ref_loss)/max(abs(ref_loss),1e-6):.2e}")
 le = relerr(logits.detach(), ref_logits)
 print(f"logits: max relerr = {le:.2e}")