fix recompute (#42128)

* fix recompute

* modify return

python/paddle/incubate/distributed/models/moe/moe_layer.py

@@ -399,7 +399,7 @@ class MoELayer(nn.Layer):
         def experts_fwd(x, fwd_expert_count, experts):
 
             if x.shape[0] == 0:
-                return paddle.empty(x.shape, x.dtype)
+                return x
             y = []
             last_index = 0
             assert isinstance(fwd_expert_count, np.ndarray)
@@ -411,7 +411,7 @@ class MoELayer(nn.Layer):
                 last_index = expert_count + last_index
             return paddle.concat(y, axis=0)
 
-        if self.recompute_interval <= 0:
+        if self.recompute_interval <= 0 or x.shape[0] == 0:
             x = experts_fwd(x, fwd_expert_count.numpy(), self.experts)
         else:
             x = _hp_recompute(experts_fwd, x,