[modeling_utils] respect original dtype in _get_resized_lm_head
Currently `resize_token_embeddings`'s `_get_resized_lm_head` doesn't preserve the original dtype when creating a new param. This PR fixes it.

The fix is identical to:

https://github.com/huggingface/transformers/blob/232822f36d49598e68e152a9ca0a6d90be6f54b5/src/transformers/modeling_utils.py#L792-L794

The problem was detected by @VictorSanh who used  https://huggingface.co/google/t5-v1_1-small w/ deepspeed z3 - this model is different from t5 as it has `"tie_word_embeddings": false`, so it goes through a different code path. 

I will have a deepspeed test covering this path once a new tiny model is added to https://huggingface.co/hf-internal-testing/ to cover this variation of t5 models.

@sgugger, @LysandreJik 