[Deepspeed] fix resize_token_embeddings
the introduction of `model.resize_token_embeddings(len(tokenizer))` in https://github.com/huggingface/transformers/commit/57c8e822f7faa1c19f9926338f21f3aab2269997#diff-09777f56cee1060a535a72ce99a6c96cdb7f330c8cc3f9dcca442b3f7768237a
uncovered a bug in Deepspeed integration. 

We have to create the new resized embedding with the same dtype as the original.
If not deepspeed will fail to initialize with: `"Model must initialized in fp16 mode for ZeRO Stage 3.` error.

This PR fixes this bug.

@sgugger 
 