Fix the bug of sequence_unpad op (#18290) (#18305)

* Use TensorCopySync for sequence_unpad op

* Fix the tensor memory alloc bug

test=release/1.5

paddle/fluid/operators/sequence_ops/sequence_unpad_op.h

@@ -34,28 +34,26 @@ class SequenceUnpadOpKernel : public framework::OpKernel<T> {
     auto* len_t = ctx.Input<LoDTensor>("Length");
     auto* out_t = ctx.Output<LoDTensor>("Out");
 
-    const int64_t* seq_len_ptr = nullptr;
+    auto& dev_ctx = ctx.template device_context<DeviceContext>();
+    framework::Tensor seq_len_cpu =
+        ctx.AllocateTmpTensor<T, DeviceContext>(len_t->dims(), dev_ctx);
     if (platform::is_gpu_place(ctx.GetPlace())) {
-      LoDTensor seq_len_cpu;
-      seq_len_cpu.Resize(len_t->dims());
-      seq_len_ptr = seq_len_cpu.mutable_data<int64_t>(platform::CPUPlace());
-      framework::TensorCopy(*len_t, platform::CPUPlace(),
-                            ctx.template device_context<DeviceContext>(),
-                            &seq_len_cpu);
+      seq_len_cpu.mutable_data<int64_t>(platform::CPUPlace());
+      framework::TensorCopySync(*len_t, platform::CPUPlace(), &seq_len_cpu);
     } else {
-      seq_len_ptr = len_t->data<int64_t>();
+      seq_len_cpu = *len_t;
     }
 
-    size_t batch_size = x_t->dims()[0];
+    const int64_t* seq_len_ptr = seq_len_cpu.data<int64_t>();
+    int64_t batch_size = len_t->dims()[0];
     std::vector<size_t> out_lod0(batch_size + 1, 0);
-    for (size_t i = 0; i < batch_size; ++i) {
-      out_lod0[i + 1] = out_lod0[i] + seq_len_ptr[i];
+    for (int64_t i = 0; i < batch_size; ++i) {
+      out_lod0[i + 1] = out_lod0[i] + static_cast<size_t>(seq_len_ptr[i]);
     }
 
     framework::LoD out_lod;
     out_lod.push_back(out_lod0);
     out_t->set_lod(out_lod);
-
     std::vector<int64_t> out_dims_vec{static_cast<int64_t>(out_lod0.back())};
     if (x_t->dims().size() == 2) {
       out_dims_vec.push_back(1);
@@ -71,8 +69,7 @@ class SequenceUnpadOpKernel : public framework::OpKernel<T> {
 
     int64_t padded_length = x_t->dims()[1];
     math::UnpaddingLoDTensorFunctor<DeviceContext, T>()(
-        ctx.template device_context<DeviceContext>(), *x_t, out_t,
-        padded_length, 0, false, math::kBatchLengthWidth);
+        dev_ctx, *x_t, out_t, padded_length, 0, false, math::kBatchLengthWidth);
   }
 };