DeepSpeed/FSDP ckpt saving utils fixes and FSDP training args fixes
# What does this PR do?
1. `_save` function saves `tokenizer` and `training_args.bin` in addition to model. 
2. This PR rearranges logic for saving model for DS and FSDP such that the above 2 things are also saved in addition to model ckpt. 
3. FIxes https://github.com/huggingface/transformers/issues/24641
_The documentation is not available anymore as the PR was closed or merged._