Privatize the sharded optimizer within the sharded DDP

[blefaudeux]

🚀 Feature

Align the OSS_ddp architecture with this proposal pytorch/pytorch#42849.
A main difference is that oss_ddp owns the construction of the sharded optimizer, it does not have to be exposed.

Motivation

• Prototype something closer to the RFC
• Bring the sharding in the same place, so that we can eventually align the sharding from the optimizer and the model parameter sharding. Right now DDP gets the full model

Pitch

Keep backward compatibilty: using OSS directly stays possible, but if targeting oss_ddp, then it owns the sharded optimizer. Eventually surface the sharding and align both

Alternatives

Keep the current approach in two seperate steps (shard DDP / shard optim), but it does not seem very future proof

Additional context