text: Title: [Bug]: optimizer state not saved
  Body: ### Describe the bug
  
  Thank you for developing and maintaining this invaluable module!
  
  We would like to save the state of the optimizer at the end of each epoch.
  The `save_optimizer_state` parameter of the `fine_tune` function seems to be designed for this purpose.
  However, the state of the optimizer is not saved even if we set `save_optimizer_state=True`.
  
  Thank you!
  
  ### To Reproduce
  
  ```python
  %pip install scipy==1.10.1 datasets transformers torch==2.0 flair==0.13.1 
  
  import torch
  import flair
  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',
                    learning_rate=5.0e-5,
                    mini_batch_size=4,
                    max_epochs=10,
                    save_optimizer_state=True,
                    save_model_each_k_epochs=1
  )
  
  checkpoint = torch.load('resources/taggers/question-classification-with-transformer/model_epoch_1.pt', map_location=flair.device)
  ```
  
  
  ### Expected behavior
  
  When `save_optimizer_state` is `true`, the checkpoint contains the state_dict of the optimizer.
  
  ### Logs and Stack traces
  
  _No response_
  
  ### Screenshots
  
  _No response_
  
  ### Additional Context
  
  _No response_
  
  ### Environment
  
  #### Versions:
  ##### Flair
  0.13.1
  ##### Pytorch
  2.0.0+cu117
  ##### Transformers
  4.40.0
  #### GPU
  True