Adds a useful high level wrapper for calling `dist.save/load` with the correct storage readers and writers.
Instead of doing:
```
DCP.save(
state_dict={...},
storage_writer=StorageWriter(...)
)
DCP.load(
state_dict={...},
storage_reader=StorageReader(...)
)
```
We can now do:
```
checkpointer = Checkpointer(...)
checkpointer.save(state_dict={...})
checkpointer.load(state_dict={...})
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/114603
Approved by: https://github.com/fegin, https://github.com/wz337
