Fix the varlen deterministic test (#1023)

Co-authored-by: moshuosha <moshuosha@qq.com>

tests/test_flash_attn.py

@@ -2459,9 +2459,9 @@ def test_flash_attn_varlen_deterministic(seqlen_q, seqlen_k, swap_sq_sk, d, caus
 
     g = torch.randn_like(out)
     if (d <= MAX_HEADDIM_SM8x or d > 224) or (is_sm80 or is_sm90):
-        dq, dk, dv = torch.autograd.grad(out, (q_unpad, k_unpad, v_unpad), g, retain_graph=True)
+        dq0, dk0, dv0 = torch.autograd.grad(out, (q_unpad, k_unpad, v_unpad), g, retain_graph=True)
         for _ in range(50):
             dq, dk, dv = torch.autograd.grad(out, (q_unpad, k_unpad, v_unpad), g, retain_graph=True)
-            assert torch.equal(dv, dv)
-            assert torch.equal(dk, dk)
-            assert torch.equal(dq, dq)
+            assert torch.equal(dv, dv0)
+            assert torch.equal(dk, dk0)
+            assert torch.equal(dq, dq0)