Added device_filter_string

```
In [1]: import dpctl

In [2]: dpctl.SyclDevice("cpu").filter_string
Out[2]: 'opencl:cpu:0'

In [3]: dpctl.SyclDevice("gpu").filter_string
Out[3]: 'level_zero:gpu:0'

In [4]: dpctl.SyclDevice("opencl:gpu").filter_string
Out[4]: 'opencl:gpu:0'

In [5]: dpctl.SyclDevice().filter_string
Out[5]: 'level_zero:gpu:0'

In [6]: dpctl.SyclDevice("0").filter_string
Out[6]: 'opencl:accelerator:0'

In [7]: dpctl.SyclDevice("1").filter_string
Out[7]: 'opencl:cpu:0'

In [8]: dpctl.SyclDevice("2").filter_string
Out[8]: 'opencl:cpu:0'

In [9]: dpctl.SyclDevice("3").filter_string
Out[9]: 'opencl:gpu:0'

In [10]: dpctl.SyclDevice("4").filter_string
Out[10]: 'level_zero:gpu:0'
```

Author: oleksandr-pavlyk

dpctl/_sycl_device.pyx

@@ -50,6 +50,7 @@ from ._backend cimport (
     DPCTLDevice_IsGPU,
     DPCTLDevice_IsHost,
     DPCTLDeviceMgr_PrintDeviceInfo,
+    DPCTLDeviceMgr_GetRelativeId,
     DPCTLFilterSelector_Create,
     DPCTLDeviceSelector_Delete,
     DPCTLDeviceSelector_Score,
@@ -78,7 +79,7 @@ from ._backend cimport (
     DPCTLDevice_GetParentDevice,
 )
 from . import backend_type, device_type
-from libc.stdint cimport uint32_t
+from libc.stdint cimport uint32_t, int64_t
 from libc.stdlib cimport malloc, free
 import warnings
 import collections
@@ -126,6 +127,34 @@ cdef list _get_devices(DPCTLDeviceVectorRef DVRef):
     return devices
 
 
+cdef str _backend_type_to_filter_string_part(DPCTLSyclBackendType BTy):
+   if BTy == _backend_type._CUDA:
+       return "cuda"
+   elif BTy == _backend_type._HOST:
+       return "host"
+   elif BTy == _backend_type._LEVEL_ZERO:
+       return "level_zero"
+   elif BTy == _backend_type._OPENCL:
+       return "opencl"
+   else:
+       return "unknown"
+
+
+cdef str _device_type_to_filter_string_part(DPCTLSyclDeviceType DTy):
+   if DTy == _device_type._ACCELERATOR:
+       return "accelerator"
+   elif DTy == _device_type._AUTOMATIC:
+       return "automatic"
+   elif DTy == _device_type._CPU:
+       return "cpu"
+   elif DTy == _device_type._GPU:
+       return "gpu"
+   elif DTy == _device_type._HOST_DEVICE:
+       return "host"
+   else:
+       return "unknown"
+
+
 cdef class SyclDevice(_SyclDevice):
     """ Python equivalent for cl::sycl::device class.
 
@@ -719,6 +748,8 @@ cdef class SyclDevice(_SyclDevice):
 
     @property
     def parent_device(self):
+        """ Parent device for a sub-device, or None for a root device.
+        """
         cdef DPCTLSyclDeviceRef pDRef = NULL
         pDRef = DPCTLDevice_GetParentDevice(self._device_ref)
         if (pDRef is NULL):
@@ -736,3 +767,27 @@ cdef class SyclDevice(_SyclDevice):
             return self.equals(<SyclDevice> other)
         else:
             return False
+
+    @property
+    def filter_string(self):
+        """ For a parent device returns a tuple (backend, device_kind, relative_id).
+            Raises an exception for sub-devices.
+        """
+        cdef DPCTLSyclDeviceRef pDRef = NULL
+        cdef DPCTLSyclBackendType BTy
+        cdef DPCTLSyclDeviceType DTy
+        cdef int64_t relId = -1
+        pDRef = DPCTLDevice_GetParentDevice(self._device_ref)
+        if (pDRef is NULL):
+            BTy = DPCTLDevice_GetBackend(self._device_ref)
+            DTy = DPCTLDevice_GetDeviceType(self._device_ref)
+            relId = DPCTLDeviceMgr_GetRelativeId(self._device_ref)
+            if (relId == -1):
+                raise TypeError("This SyclDevice is not a root device")
+            br_str = _backend_type_to_filter_string_part(BTy)
+            dt_str = _device_type_to_filter_string_part(DTy)
+            return ":".join((br_str, dt_str, str(relId)))
+        else:
+            # this a sub-device, free it, and raise an exception
+            DPCTLDevice_Delete(pDRef)
+            raise TypeError("This SyclDevice is not a root device")