[DCP][state_dict] Implement pin_memory and shared_memory copy for _offload_state_dict_to_cpu (#120378)
**Summary**
This PR extend `_offload_state_dict_to_cpu` to accept a `cpu_offload_state_dict` argument. If `cpu_offload_state_dict` is not None, `_offload_state_dict_to_cpu` will use `copy_` to copy the GPU data to the CPU tensors. This allows users to pass a pin_memory or share_memory version of `cpu_offload_state_dict`.
This PR also adds `_create_cpu_state_dict` to allow users to easily create a pin_memory or share_memory cpu state_dict.
**Performance improvement**
```
# The micro-benchmark has a source state_dict with 150 tensors, and each tensor is 50MB.
# The micro-benchmark is run on a H100 machine with PCIe 5
cpu_state_dict_2 = _create_cpu_state_dict(state_dict, pin_memory=True)
cpu_state_dict_3 = _create_cpu_state_dict(state_dict, share_memory=True)
# GPU->CPU memory: 4.6556 seconds
cpu_state_dict = _offload_state_dict_to_cpu(state_dict)
# GPU->pin memory: 0.1566 seconds
_offload_state_dict_to_cpu(state_dict, cpu_offload_state_dict=cpu_state_dict_2)
# GPU->shared memory: 0.5509 seconds (variation is quite large)
_offload_state_dict_to_cpu(state_dict, cpu_offload_state_dict=cpu_state_dict_3)
# GPU->pin memory->shared memory: 0.2550 seconds
_offload_state_dict_to_cpu(state_dict, cpu_offload_state_dict=cpu_state_dict_2)
_offload_state_dict_to_cpu(cpu_state_dict_2, cpu_offload_state_dict=cpu_state_dict_3)
```
Differential Revision: [D54045845](https://our.internmc.facebook.com/intern/diff/D54045845/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120378
Approved by: https://github.com/LucasLLC
