Fix `is_optimum_neuron_available`
# What does this PR do?

#23163 introduced a new way for checking which seems to not work in this case. 
I get this when running `optimum-neuron`:

```
Please use the TrainiumTrainer from optimum[neuron] instead of the Transformers library to perform training on AWS Trainium instances. More information here: https://github.com/huggingface/o
ptimum-neuron  
```

It happens [here](https://github.com/huggingface/transformers/blob/main/src/transformers/training_args.py#L82).
Basically it means that `optimum-neuron` is found to not be available, this PR fixes that.
_The documentation is not available anymore as the PR was closed or merged._<|||||>No, here the check for `_is_package_available("optimum.neuron")` is done at "runtime" not "import time". 
For some reason `optimum.neuron` is not available at import time of this file but is available later... 
Maybe this is due to the fact that `optimum` is a namespace package, not sure.<|||||>Yes, I will investigate. If I find a fix on the other side, will revert back to the original version here.<|||||>I'm fine with having the workaround :-) Just flagging this for you to look at when you have some time.<|||||>Failure is due to a library pinned on main, so merging.