Fix "load_model_sharded" when "--save_optimizer" enabled

[abhika-m]

Fixing "key error" when loading optimizer with sharded checkpoint loading

Potentially Fixes # 127

Feature/Issue validation/testing

• Enabled "save_optimizer" then ran training with StateDictType.SHARDED_STATE_DICT for one epoch then loaded from checkpoint

1. Ran for one epoch:

python ./llama_finetuning.py ... --save_optimizer

2. Loaded from checkpoint created:

load_model_sharded(model, rank, cfg)

3. Loading checkpoints resulted in:

checkpoint key len = 1 and 
keys =  dict_keys(['model'])

If "--save_optimizer" is enabled then checkpoint key length should be "2" and keys should be "dict_keys(['model', 'optim'])

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