lfw bug fix

Hi,

When trying to get all_samples() in lfw dataset, I faced with the following error:

>> from bob.bio.face.config.database.lfw_unrestricted import database
>> database.all_samples()
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-91-392e908f49b8> in <module>
      1 from bob.bio.face.config.database.lfw_unrestricted import database
----> 2 database.all_samples()

/idiap/temp/hotroshi/anaconda3/envs/my_bob_2021/lib/python3.8/site-packages/bob/bio/base/pipelines/vanilla_biometrics/legacy.py in all_samples(self, groups)
    237         """
    238         valid_groups = convert_names_to_highlevel(
--> 239             self.database.groups(),
    240             low_level_names=["world", "dev", "eval"],
    241             high_level_names=["train", "dev", "eval"],

/idiap/temp/hotroshi/anaconda3/envs/my_bob_2021/lib/python3.8/site-packages/bob/bio/base/database/database.py in groups(self, protocol)
    307           If you do not have protocols defined, just ignore this field.
    308         """
--> 309         raise NotImplementedError("This function must be implemented in your derived class.")
    310 
    311 

NotImplementedError: This function must be implemented in your derived class.

It is solved in this MR (thanks @amohammadi)