stack support one input

mindspore/lite/nnacl/fp32/stack.c

@@ -67,3 +67,7 @@ void DoStackInt32(const int32_t *const *inputs, size_t input_num, int *in_shape,
     in_offset += copy_num;
   }
 }
+
+void DoStackOneInput(const int8_t *input, int8_t *output, size_t data_size) {
+  memcpy(output, input, data_size);
+}

mindspore/lite/nnacl/fp32/stack.h

@@ -29,6 +29,7 @@ extern "C" {
 void DoStack(const float *const *inputs, size_t input_num, int *in_shape, size_t shape_size, int axis, float *output);
 void DoStackInt32(const int32_t *const *inputs, size_t input_num, int *in_shape, size_t shape_size, int axis,
                   int32_t *output);
+void DoStackOneInput(const int8_t *input, int8_t *output, size_t data_size);
 #ifdef __cplusplus
 }
 #endif

mindspore/lite/src/ops/stack.cc

@@ -58,7 +58,7 @@ int Stack::UnPackToFlatBuilder(const schema::Primitive *primitive, flatbuffers::
 
 namespace {
 constexpr int kStackOutputNum = 1;
-constexpr int kStackMinInputNum = 2;
+constexpr int kStackMinInputNum = 1;
 }  // namespace
 int Stack::InferShape(std::vector<tensor::Tensor *> inputs, std::vector<tensor::Tensor *> outputs) {
   MS_ASSERT(this->primitive_ != nullptr);

mindspore/lite/src/runtime/kernel/arm/fp32/stack.cc

@@ -48,6 +48,12 @@ int StackCPUKernel::Run() {
     return ret;
   }
   size_t inputs_num = in_tensors_.size();
+  auto input0 = in_tensors_[0];
+  if (inputs_num == 1) {
+    auto *output_data = reinterpret_cast<int8_t *>(out_tensors_[0]->Data());
+    DoStackOneInput(reinterpret_cast<const int8_t *>(input0->Data()), output_data, input0->Size());
+    return RET_OK;
+  }
   auto input0_shape = in_tensors_[0]->shape();
   if (in_tensors_[0]->data_type() == kNumberTypeFloat32 || in_tensors_[0]->data_type() == kNumberTypeFloat) {
     auto *output_data = reinterpret_cast<float *>(out_tensors_[0]->Data());