[Composable APIs] Add composable API `fully_shard` deprecation warning (#120929)
`fully_shard`(https://github.com/pytorch/pytorch/blob/main/torch/distributed/_composable/fsdp/fully_shard.py) will be used by new FSDP2 and we want to add a deprecation warning to the existing composable API's `fully_shard`(https://github.com/pytorch/pytorch/blob/main/torch/distributed/_composable/fully_shard.py#L40).
Planned release schedule is as follows https://dev-discuss.pytorch.org/t/release-cadence-for-year-2023-2024/1557:
Minor Version | Release branch cut | Release date | First patch release date | Second patch release date
-- | -- | -- | -- | --
2.3 | Mar 2024 | Apr 2024 | May 2024 | Jun 2024
2.4 | May 2024 | Jul 2024 | Aug 2024 | Sep 2024
2.5 | Aug 2024 | Oct 2024 | Nov 2024 | Dec 2024
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120929
Approved by: https://github.com/awgu
