Updated checkpoint support for Sagemaker Model Parallel
Author: cavdard
Pull request: yes

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This PR updates SMP checkpoint support. With these changes SMP optimizer state checkpoints will be saved partially while SMP model weights will be saved in full. Since weights are saved in full,  checkpoint behavior will be compatible with  `save_pretrained` and `shard_checkpoint`. 


- Uses `local_state_dict()` with partial optimizer state saving.
- Uses `smp.save` optimizer state saving for SMP.
- Uses `smp.load `when loading optimizer state saving for SMP. 
- Reorders weight loading to happen after wrapping of model for SMP. 
- Updated checks for the existence of optimizer checkpoint files since smp partial checkpoints contain postfixes in addition to filename(example: `filename_0_0` or `filename_0_0_0`).
- adds `load_best_model_at_end` support for SMP

This PR is created based on the feedback from [previous PR on partial checkpoint support for SMP:](https://github.com/huggingface/transformers/pull/16950)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

> Thanks a lot for your PR! I've left some refactoring suggestions to avoid duplicating code.

Thank you very much for reviewing. I updated the PR. 

Updated based on you suggestions.