Updates in Trainer to support partial checkpointing for SM Model Parallel library 
Author: cavdard
Pull request: yes

# What does this PR do?

- Adds 2 new training args(`smp_save_partial` and `smp_load_partial`) to support partial checkpointing with SMP
- Uses the right ranks for partial checkpoint saving in should_save.
- Uses `local_state_dict()` with partial checkpoint saving.
- Uses `smp.save` instead of `torch.save` when partial checkpoint saving is enabled. 
- Uses `smp.load` instead of `torch.load` when partial checkpoint loading is enabled.  Reorders partial checkpoint loading to happen after wrapping of model, since `smp.load` can only load to a smp model.
- Updated checks for the existence of checkpoint files since smp partial checkpoints contain postfixes in addition to filename(example: filename_0_0 or filename_0_0_0).
- `smp_gather` is causing increased memory usage on GPU0 when tensor parallelism is enabled. Switches to `distributed_concat` for ddp. 
- adds `load_best_model_at_end` support for SMP.

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16314). All of your documentation changes will be reflected on that endpoint.

Hey! It seems a bad rebase/merge happened on your PR. Usually, closing this PR and opening a new one from the same branch solves the problem.

Closing this PR. 
Created a new [PR](https://github.com/huggingface/transformers/pull/16734)


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