change TokenClassificationTask class methods to static methods
Author: donchev7
Pull request: yes

Since we do not require self in the class methods of TokenClassificationTask we should probably switch to static methods. Also, since the class TokenClassificationTask does not contain a constructor it is currently unusable as is. By switching to static methods this fixes the issue of having to document the intent of the broken class.

Also, since the get_labels and read_examples_from_file methods are ought to be implemented. Static method definitions are unchanged even after inheritance, which means that it can be overridden, similar to other class methods.

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Comments:
But why doesn't this PR trigger a CI test? @LysandreJik 

I have no idea, this is the second time it happens. I sent an empty commit on your branch to trigger the build @donchev7.

Pinging @stefan-it for review.