fix GeneratorDataset join error

mindspore/dataset/engine/datasets.py

@@ -3450,6 +3450,7 @@ class SamplerFn:
         self.num_worker = num_worker
         self.multi_process = multi_process
         self.joined = False
+        self.ppid = os.getpid()
         # Event for end of epoch
         if multi_process is True:
             self.eof = multiprocessing.Event()
@@ -3508,11 +3509,12 @@ class SamplerFn:
             yield tuple([np.array(x, copy=False) for x in result])
 
     def _stop_subprocess(self):
+        # Only the main process can call join
+        if self.joined is False and self.ppid == os.getpid():
             self.eof.set()
-        if self.joined is False:
+            self.joined = True
             for w in self.workers:
                 w.join()
-            self.joined = True
 
     def __del__(self):
         self._stop_subprocess()