GH-2426: multi-label tars

[alanakbik]

Closes #2426 by adding the possibility to set confidence thresholds on multi-label prediction in TARS, and setting whether a problem is single-label or multi-label:

from flair.models import TARSClassifier
from flair.data import Sentence

# 1. Load our pre-trained TARS model for English
tars: TARSClassifier = TARSClassifier.load('tars-base')

# switch to a multi-label task (emotion detection)
tars.switch_to_task('GO_EMOTIONS')

# sentence with two emotions
sentence = Sentence("I am happy and sad")

# predict normally
tars.predict(sentence)
print(sentence)

# predict with lower label threshold (you can set this to 0. to get all labels)
tars.predict(sentence, label_threshold=0.01)
print(sentence)

# predict and enforce a single-label prediction
tars.predict(sentence, label_threshold=0.01, multi_label=False)
print(sentence)