[FSDP()] Rename to `fully_shard()` and move to `_composable/` (#88233)
After internal discussion, we are currently preferring `fully_shard()` as the name of the composable FSDP API.
- `FullyShardedDataParallel` (FSDP) has existing brand value, so the chosen name should try to preserve that. We think this takes precedence over the fact that composable FSDP may encompass than just the ZeRO-3 approach of _fully sharding_.
- Given the refactoring efforts, it would also not be challenging to create a new frontend API like `hybrid_shard()` that calls into the same underlying initialization and runtime except for a different `ShardingStrategy`. In other words, we do not have to coalesce all sharding strategies under `fully_shard()`.
- The other composable APIs are verbs (`replicate()`, `checkpoint()`), so the chosen name should be a verb.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88233
Approved by: https://github.com/mrshenli
