add reduce transformer vocab plugin

[helpmefindaname]

No description provided.

[alanakbik]

Looks great, thanks for adding this @helpmefindaname!

Tested locally and got a 25% increase in training speed for this script with reduce_transformer_vocab=True compared to reduce_transformer_vocab=False:

from flair.data import Corpus
from flair.datasets import TREC_6
from flair.embeddings import TransformerDocumentEmbeddings
from flair.models import TextClassifier
from flair.trainers import ModelTrainer

# 1. get the corpus
corpus: Corpus = TREC_6()

# 2. what label do we want to predict?
label_type = "question_class"

# 3. create the label dictionary
label_dict = corpus.make_label_dictionary(label_type=label_type)

# 4. initialize transformer document embeddings (many models are available)
document_embeddings = TransformerDocumentEmbeddings("distilbert-base-uncased", fine_tune=True)

# 5. create the text classifier
classifier = TextClassifier(document_embeddings, label_dictionary=label_dict, label_type=label_type)

# 6. initialize trainer
trainer = ModelTrainer(classifier, corpus)

# 7. run training with fine-tuning
trainer.fine_tune(
    "resources/taggers/question-classification-with-transformer",
    reduce_transformer_vocab=True,  # set this to False for slow version
    learning_rate=5.0e-5,
    mini_batch_size=4,
)