Improve task api code quality
Looks good thanks, what do you think @lewtun ? thanks for including the lazy `ClassLabel` class @mariosasko ! from my side this LGTM!
Improves the code quality of the `TaskTemplate` dataclasses.

Changes:
* replaces `return NotImplemented` with raise `NotImplementedError` 
* replaces `sorted` with `len` in the uniqueness check 
* defines `label2id` and `id2label` in the `TextClassification` template as properties
* replaces the `object.__setattr__(self, attr, value)` syntax with (IMO nicer) `self.__dict__[attr] = value`