Found multiple skeletons after running inference

[Desvino]

Hi! Thanks for your help!

Bug description

We first encountered this bug when we tried to use relabelled instances to run training.

When performing a single animal project, it turned to

ValueError: Labels.skeleton can only be used when there is only a single skeleton saved in the labels. Use Labels.skeletons instead. 

But it could run when we restarted Sleap.

However, when performing a multiple animal top-down project, it turned to

Traceback (most recent call last):
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\Scripts\sleap-train-script.py", line 33, in <module>
    sys.exit(load_entry_point('sleap==1.3.4', 'console_scripts', 'sleap-train')())
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\lib\site-packages\sleap\nn\training.py", line 2014, in main
    trainer.train()
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\lib\site-packages\sleap\nn\training.py", line 924, in train
    self.setup()
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\lib\site-packages\sleap\nn\training.py", line 910, in setup
    self._setup_model()
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\lib\site-packages\sleap\nn\training.py", line 727, in _setup_model
    base_example = next(iter(base_pipeline.make_dataset()))
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\lib\site-packages\tensorflow\python\data\ops\iterator_ops.py", line 800, in __next__
    return self._next_internal()
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\lib\site-packages\tensorflow\python\data\ops\iterator_ops.py", line 786, in _next_internal
    output_shapes=self._flat_output_shapes)
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\lib\site-packages\tensorflow\python\ops\gen_dataset_ops.py", line 2844, in iterator_get_next
    _ops.raise_from_not_ok_status(e, name)
  File "C:\Users\MiaoLab-Guest\anaconda3\envs\sleap\lib\site-packages\tensorflow\python\framework\ops.py", line 7107, in raise_from_not_ok_status
    raise core._status_to_exception(e) from None  # pylint: disable=protected-access
tensorflow.python.framework.errors_impl.InvalidArgumentError: indices[0] = 2 is not in [0, 1)
         [[{{node GatherV2}}]] [Op:IteratorGetNext]

In this case, it couldn't run when we restarted Sleap.
I think it is a similar issue to #713 and #1090.
When we tried methods you provided in #713 , we found that there were 3 skeletons in our project. These 3 skeletons were totally the same. After deleting the other skeletons, we could run training.
So I think multiple skeletons appeared as followed:

• Add videos into new project
• Run inference using previous model
• Edit instances
• Run training
• See error

We want to know how can we avoid this bug.Thank you very much!

Your personal set up

• OS: Windows 10

• Version(s): Sleap v1.3.4

• SLEAP installation method (listed here):
  • Conda from package