Guard DeepSpeed imports
Author: lewtun
Pull request: yes

# What does this PR do?

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This PR follows a similar approach as the one taken in `accelerate` (https://github.com/huggingface/accelerate/pull/1963) to guard the DeepSpeed imports so that we only call `import deepspeed` when it's needed. The rationale for this is that whenever we run `import deepspeed`, CUDA is initialised which is (a) slow and (b) interferes with the vllm server when DP=8 and TP=1 (see https://github.com/vllm-project/vllm/issues/17079)

Related to https://github.com/huggingface/trl/pull/3351

Gently pinging @SunMarc for review 🤗 

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