fast_dev_run not supported

[finnmayhew]

Describe the bug
When model.fit() is run with fast_dev_run=True, the code crashes.

fast_dev_run=True disables model checkpoints, but easy_model.py creates the ModelCheckpoint callback regardless (L134). When the same file later checks for the model checkpoint (L181), it doesn't find a file and crashes.

To Reproduce
Run fit() from EasySyntax with fast_dev_run=True passed in (interpreted as one of the trainer_kwargs).

Expected behavior
Expected easy_model.py would not create the ModelCheckpoint callback given fast_dev_run=True.

Full traceback
Please include the full error message to allow for debugging

Traceback (most recent call last):
  File "/mnt/home/mayhewfi/DP/event-nn/python/Train.py", line 334, in <module>
    main(args)
  File "/mnt/home/mayhewfi/DP/event-nn/python/Train.py", line 282, in main
    model.fit(
  File "/mnt/ffs24/home/mayhewfi/DP/graphnet/src/graphnet/models/easy_model.py", line 182, in fit
    torch.load(
  File "/mnt/home/mayhewfi/DP/event-nn-env/lib/python3.11/site-packages/torch/serialization.py", line 998, in load
    with _open_file_like(f, 'rb') as opened_file:
         ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/home/mayhewfi/DP/event-nn-env/lib/python3.11/site-packages/torch/serialization.py", line 445, in _open_file_like
    return _open_file(name_or_buffer, mode)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/home/mayhewfi/DP/event-nn-env/lib/python3.11/site-packages/torch/serialization.py", line 426, in __init__
    super().__init__(open(name, mode))
                     ^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: ''