Disable tokenizers parallelism in multiprocessed map
It was reported in #620 that using multiprocessing with a tokenizers shows this message:
```
The current process just got forked. Disabling parallelism to avoid deadlocks...
To disable this warning, please explicitly set TOKENIZERS_PARALLELISM=(true | false)
```
This message is shown when TOKENIZERS_PARALLELISM is unset.
Moreover if it is set to `true`, then the program just hangs.

To hide the message (if TOKENIZERS_PARALLELISM is unset) and avoid hanging (if TOKENIZERS_PARALLELISM is `true`), then I set TOKENIZERS_PARALLELISM to `false` when forking the process. After forking is gets back to its original value.

Also I added a warning if TOKENIZERS_PARALLELISM was `true` and is set to `false`:
```
Setting TOKENIZERS_PARALLELISM=false for forked processes.
```

cc @n1t0 