don't load state_dict twice when using low_cpu_mem_usage in from_pretrained
# What does this PR do?

In `from_pretrained` the `state_dict` is loaded twice when `low_cpu_mem_usage` is `True`,  which is not required since the `state_dict` is already loaded before as we can here.
https://github.com/huggingface/transformers/blob/21decb7731e998d3d208ec33e5b249b0a84c0a02/src/transformers/modeling_utils.py#L1795-L1797

So, it's fine to remove it there because when is_sharded=False, the state_dict is loaded at line 1797
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