fix autotune (#42249)

fix autotune in reader.py
bug原因：创建dataloader的使用直接使用原始dataloader 等于的方式: new_dataloader = old_dataloader，属于深拷贝，导致后续修改new_dataloader的dataset用于num_workers选择时，会造成old_dataloader的dataset同步被修改。导致模型运行的dataset与实际dataset不相等

python/paddle/fluid/reader.py

@@ -19,6 +19,7 @@ import numpy as np
 import threading
 import paddle
 import time
+import copy
 
 from .framework import Program, Variable, program_guard, default_main_program, default_startup_program, _non_static_mode, cpu_places, _current_expected_place, _in_eager_without_dygraph_check
 from .executor import global_scope
@@ -214,7 +215,7 @@ class AuToTune(object):
         return sub_dataset
 
     def get_autotune_loader(self):
-        loader = self.loader
+        loader = copy.copy(self.loader)
         batch_size = self.loader.batch_sampler.batch_size
         if isinstance(self.loader.batch_sampler,
                       paddle.io.DistributedBatchSampler):