Fix upcycling state dict conversion for mixed dense/MoE models

[rkteddy]

What does this PR do ?

Fix upcycling pipeline for mixed dense/MoE models (i.e., --moe-layer-freq is set with --moe-use-upcycling):

1. Fix load_kwargs key in training.py: load_checkpoint expects ddp_model but the upcycling path passed model, causing a KeyError.
2. Fix upcycling_utils.py: in a mixed model, dense layers have no expert parameters, so the converted keys are absent from the MoE state dict. Add an existence check in
   _convert_key_value and _expand_key_value to skip such keys.

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[yanring]

LGTM, thanks!