Feat: enhance golden validation error output with mismatch details

- Add detailed mismatch information to golden validation error messages
- Show first 20 mismatched elements with indices and actual/expected values
- Use vectorized operations for better performance
- Improve debugging experience for test failures

Author: sjduan

simpler_setup/code_runner.py

@@ -933,12 +933,25 @@ def _compare_with_golden(
             if not torch.allclose(actual, expected, rtol=self.rtol, atol=self.atol):
                 # Find mismatches for better error reporting
                 close_mask = torch.isclose(actual, expected, rtol=self.rtol, atol=self.atol)
-                mismatches = (~close_mask).sum().item()
                 total = actual.numel()
+                mismatch_indices = torch.where(~close_mask.flatten())[0]
+                n_show = min(20, mismatch_indices.numel())
+                flat_actual = actual.flatten()
+                flat_expected = expected.flatten()
+
+                # Efficiently extract values
+                show_indices = mismatch_indices[:n_show]
+                actual_vals = flat_actual[show_indices].tolist()
+                expected_vals = flat_expected[show_indices].tolist()
+                detail_str = "\n".join(
+                    f"  [{idx}] actual={act}, expected={exp}"
+                    for idx, act, exp in zip(show_indices.tolist(), actual_vals, expected_vals)
+                )
                 raise AssertionError(
                     f"Output '{name}' does not match golden.\n"
-                    f"Mismatched elements: {mismatches}/{total}\n"
-                    f"rtol={self.rtol}, atol={self.atol}"
+                    f"Mismatched elements: {mismatch_indices.numel()}/{total}\n"
+                    f"rtol={self.rtol}, atol={self.atol}\n"
+                    f"First {n_show} mismatches:\n{detail_str}"
                 )
 
             matched = torch.isclose(actual, expected, rtol=self.rtol, atol=self.atol).sum().item()