Introduce shard-merging util for FSDP (#2772)
* Initial commit
* Now to test
* Store false
* Slight tweaks
* Fix naming
* Got it all working with tests
* Use not for safetensors arg
* rm change
* Add docs
* Adjust based on Marc's feedback
* Specify just weights
* Update tests to include CLI and swap namings
* Fin
* Rm unused
* Rm again
