Created by: JiayiFeng

current LoDTensor:

message LoDTensorDesc {
  required DataType data_type = 1;
  repeated int32 dims = 2; // [UNK, 640, 480] is saved as [-1, 640, 480]
  optional int32 lod_level = 3 [ default = 0 ];
}

Our Tensor::dims_ is using int64, so here we shall also use int64.