Fix dataloader._shutdown_workers if not all workers are started (#23761)
Summary:
Otherwise you may see errors like
```
Exception ignored in: <function _MultiProcessingDataLoaderIter.__del__ at 0x000001F99F5CB9D8>
Traceback (most recent call last):
File "C:\Users\Divyansh J\Anaconda3\envs\pytorch\lib\site-packages\torch\utils\data\dataloader.py", line 883, in __del__
self._shutdown_workers()
File "C:\Users\Divyansh J\Anaconda3\envs\pytorch\lib\site-packages\torch\utils\data\dataloader.py", line 860, in _shutdown_workers
if self.workers_status[worker_id]:
IndexError: list index out of range
```
e.g. https://discuss.pytorch.org/t/how-to-construct-dataset-with-iterator-for-multi-process-dataloader/49612/5
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23761
Differential Revision: D16644687
Pulled By: soumith
fbshipit-source-id: a60e847431264525079456ff422317af1ac2be4b
