I see you’re instantiating the model twice, one time providing num_labels, one time not. Note that any model in HuggingFace Transformers will have num_labels = 2 if not specified.
In any case, you should initialize the model only once, as follows:
from transformers import DistilBertForTokenClassification

model = DistilBertForTokenClassification.from_pretrained('distilbert-base-cased', num_labels=len(unique_tags))