code: def test_merge_shards(self):
      from bigdl.orca.data.utils import spark_df_to_pd_sparkxshards
      from pyspark.sql import SparkSession
  
      spark = SparkSession.builder.getOrCreate()
      df1 = spark.createDataFrame(
          [(1, 2.0), (2, 3.0), (3, 5.0), (4, 1.0)], schema=["a", "b"]
      )
      df2 = spark.createDataFrame(
          [(1, 7), (2, 8), (4, 9), (5, 9)], schema=["a", "c"]
      )
      data_shard1 = spark_df_to_pd_sparkxshards(df1)
      data_shard2 = spark_df_to_pd_sparkxshards(df2)
      merged_shard = data_shard1.merge(data_shard2, on="a")
      merged_shard_df = merged_shard.to_spark_df()
      assert len(merged_shard) == 3 and merged_shard_df.columns == [
          "a",
          "b",
          "c",
      ]
  
imports: import os.path
  import pytest
  from unittest import TestCase
  import bigdl.orca.data
  import bigdl.orca.data.pandas
  from bigdl.orca.data import SharedValue
  from bigdl.dllib.nncontext import *
  from bigdl.orca.data.transformer import *
  from bigdl.orca import init_orca_context
label_desc: Use image node_id 18 for calling the TestSparkXShards obj's underlying member method code with example usage: obj.test_merge_shards() without return types