A warning is raised when using DistributedDataParallel of PyTorch
 When I train huggingface-transformers model in multi-cards and multi-machines using DistributedDataParallel (DDP) of PyTorch, a warning is always output at each epoch:
```
huggingface/tokenizers: The current process just got forked, after parallelism has already been used. Disabling parallelism to avoid deadlocks...
To disable this warning, you can either:
- Avoid using `tokenizers` before the fork if possible

- Explicitly set the environment variable TOKENIZERS_PARALLELISM=(true | false)
```
Does this warning degrade the performace of parallel training? How to fix it? Thanks
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