Update requirements.txt

[xu-song]

No module named 'best_download' while training

$  python ./deepy.py ./train.py ./configs/125M.yml ./configs/local_setup.yml
...
Traceback (most recent call last):
  File "./train.py", line 20, in <module>
    from eval_tasks import run_eval_harness
  File "/gpt-neox/eval_tasks/__init__.py", line 15, in <module>
    import best_download
ModuleNotFoundError: No module named 'best_download'
    from megatron.training import pretrain
  File "/gpt-neox/megatron/training.py", line 58, in <module>
    from .eval_adapter import EvalHarnessAdapter, run_eval_harness
  File "/gpt-neox/eval_tasks/eval_adapter.py", line 16, in <module>
    from eval_tasks import run_eval_harness
  File "/gpt-neox/eval_tasks/__init__.py", line 15, in <module>
    import best_download
ModuleNotFoundError: No module named 'best_download'
    from .eval_adapter import EvalHarnessAdapter, run_eval_harness
  File "/gpt-neox/eval_tasks/eval_adapter.py", line 16, in <module>
    import best_download
ModuleNotFoundError: No module named 'best_download'