fix accelerator prepare during eval only mode
# What does this PR do?
1. As mentioned in https://github.com/huggingface/transformers/pull/23957#issuecomment-1573810900, currently the accelerate `prepare` method is happening only during training loop. If the user is directly doing `eval`/`predict` without the training loop, the model isn't prepared leading to wrong behaviour. This PR is aimed at fixing it.
2. Should be merged after https://github.com/huggingface/accelerate/pull/1540
_The documentation is not available anymore as the PR was closed or merged._<|||||>The thing is that mixed precision application for eval only mode won't work unless we prepare model