fix data structure for nebula datasource (#30)

Author: Nicole00

nebula-algorithm/src/main/scala/com/vesoft/nebula/algorithm/reader/DataReader.scala

@@ -50,8 +50,17 @@ class NebulaReader(spark: SparkSession, configs: Configs, partitionNum: String)
         .build()
       if (dataset == null) {
         dataset = spark.read.nebula(config, nebulaReadEdgeConfig).loadEdgesToDF()
+        if (weights.nonEmpty) {
+          dataset = dataset.select("_srcId", "_dstId", weights(i))
+        }
       } else {
-        dataset = dataset.union(spark.read.nebula(config, nebulaReadEdgeConfig).loadEdgesToDF())
+        var df = spark.read
+          .nebula(config, nebulaReadEdgeConfig)
+          .loadEdgesToDF()
+        if (weights.nonEmpty) {
+          df = df.select("_srcId", "_dstId", weights(i))
+        }
+        dataset = dataset.union(df)
       }
     }
     dataset