make loss cond. on label while using Stirling approximation (#56992)

python/paddle/nn/functional/loss.py

@@ -1594,9 +1594,9 @@ def poisson_nll_loss(
             + 0.5 * paddle.log(2 * math.pi * label)
         )
         loss_out += paddle.where(
-            stirling_approx <= 1,
-            paddle.zeros_like(stirling_approx),
+            label > 1,
             stirling_approx,
+            paddle.zeros_like(stirling_approx),
         )
     if reduction == 'mean':
         loss_out = paddle.mean(loss_out)

test/legacy_test/test_poisson_nll_loss.py

@@ -51,7 +51,9 @@ def ref_poisson_nll_loss(
         stirling_approx = (
             label * np.log(label) - label + 0.5 * np.log(2 * np.pi * label)
         )
-        loss_out += np.where(stirling_approx <= 1, 0, stirling_approx)
+        loss_out += np.where(
+            label > 1, stirling_approx, np.zeros_like(stirling_approx)
+        )
 
     if reduction == 'none':
         return loss_out