text: Title: [Bug]: ModuleNotFoundError: 'flair.trainers.plugins.functional' on git-installed master
  Body: ### Describe the bug
  
  When installing the flair master branch via pypi & git, we get an ModuleNotFound error.
  
  ### To Reproduce
  
  ```python
  exec("pip install git+https://github.com/flairNLP/flair.git")
  from flair.models import TARSClassifier
  ```
  
  
  ### Expected behavior
  
  I can import any module and use flair normally.
  
  ### Logs and Stack traces
  
  ```stacktrace
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "C:\Users\bened\anaconda3\envs\steepmc_clf\lib\site-packages\flair\__init__.py", line 28, in <module>
      from . import (  # noqa: E402 import after setting device
    File "C:\Users\bened\anaconda3\envs\steepmc_clf\lib\site-packages\flair\trainers\__init__.py", line 2, in <module>
      from .trainer import ModelTrainer
    File "C:\Users\bened\anaconda3\envs\steepmc_clf\lib\site-packages\flair\trainers\trainer.py", line 19, in <module>
      from flair.trainers.plugins import (
    File "C:\Users\bened\anaconda3\envs\steepmc_clf\lib\site-packages\flair\trainers\plugins\__init__.py", line 2, in <module>
      from .functional.amp import AmpPlugin
  ModuleNotFoundError: No module named 'flair.trainers.plugins.functional'
  ```
  
  
  ### Screenshots
  
  _No response_
  
  ### Additional Context
  
  I suppose this happens due to missing `__init__.py` files in https://github.com/flairNLP/flair/tree/master/flair/trainers/plugins/functional and https://github.com/flairNLP/flair/tree/master/flair/trainers/plugins/loggers 
  
  leading to those folders not being recognized as modules and therefore won't be found/installed as code.
  
  ### Environment
  
  #### Versions:
  ##### Pytorch
  2.0.0+cu117
  ##### flair
  `ModuleNotFoundError`
  ##### Transformers
  4.28.1
  #### GPU
  True
  