What does this PR do?

Adds DeepSpeed ZeRO 3 to the DeepSpeed plugin. Due to the nature of the plugin, we've had to make a few changes summarized below.

• Plugin has to manage saving logic as DeepSpeed only supports non-elastic checkpointing; i.e if you train on 8 GPUs, you must load the model on 8 GPUs. This will be addressed over the coming weeks as PRs are merged into DeepSpeed
• We've introduced a sharded context manager to create layers and shard them instantly.
• DeepSpeed in ZeRO 3 has to wrap the model with optimizers even in inference. This is a limitation of DeepSpeed where the sharding logic is tied to the Optimizer, hence we only get proper sharding logic when an optimizer is passed through. Overall this means we have to allocate some memory to optimizer state (which is already done within training).

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