[F1Score] average should be None by default

[cgarciae]

In the documentation it says that the average parameters is None by default but the signature has it as a required argument. See this line:

addons/tensorflow_addons/metrics/f_scores.py

Line 338 in 40de2b9

def __init__(self, num_classes, average, name='f1_score',

[Liu-Da]

I also found the same problem.

[seanpmorgan]

Please consider submitting a PR to fix this. Thanks!

[seanpmorgan]

Nevermind looks like it was fixed by #502. Closing.