code: def test_collate():
      def collate_fn(batch):
          return pa.table({"bar": [1] * 2})
  
      batches = [
          Batch(i, data)
          for i, data in enumerate(
              block_generator(num_rows=2, num_blocks=2)
          )
      ]
      batch_iter = collate(batches, collate_fn=collate_fn)
  
      for i, batch in enumerate(batch_iter):
          assert batch.batch_idx == i
          assert batch.data == pa.table({"bar": [1] * 2})
  
imports: import threading
  import time
  import numpy
  import pandas
  import pyarrow
  import pytest
  import ray
  from ray.data._internal.block_batching.interfaces import Batch
  from ray.data._internal.block_batching.util import _calculate_ref_hits, blocks_to_batches, collate, finalize_batches, format_batches, resolve_block_refs
  from ray.data._internal.util import Queue, make_async_gen
label_desc: Use image node_id 5 for calling a global function with example usage: test_collate() and returns: pa