[DCP] Adds main in format utils (#120128)
Adds main in format utils. Usage:
`python -m torch.distributed.checkpoint.format_utils dcp_to_torch dcp_dir torch_file.pt`
or
`python -m torch.distributed.checkpoint.format_utils torch_to_dcp torch_file.pt dcp_dir`
Differential Revision: [D53791355](https://our.internmc.facebook.com/intern/diff/D53791355/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120128
Approved by: https://github.com/fegin, https://github.com/wz337
