[DTensor] Allow DTensor support cuda-like device (#102468)
Allow DTensor support cuda-like device, fix https://github.com/pytorch/pytorch/issues/102442
Currently, DTensor supports cuda and cpu. There are other efforts to make DTensor support third-party devices, for example https://github.com/pytorch/pytorch/pull/101914 and https://github.com/pytorch/pytorch/issues/101911. However, this support only extends a portion of third-party devices and is no good support for third-party cuda-like devices. Therefore, we would like to extend DTensor to support cuda-like devices, after all, cuda is so popular!
1. Similar to what is done here, we need to initialize the communication backend for the device set by DeviceMesh. So `_default_backend_for_device` is added to `Backend`. It is worth noting that when we register a new backend for a device other than cpu and cuda, we also need to add a new default backend for this device.
2. Adding `_device_handle` to `DeviceMesh` for cuda-like devices, similar to what is set in FSDP. When `_device_handle` is not None, the device has similar behavior to `cuda`. In this way, functions like `torch.cuda.device_count()` need to be modified to `device_mesh._device_handle.device_count()`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102468
Approved by: https://github.com/wanchaol
