Added get_device_cached_queue utility function

This function caches queues by (context, device) key.
The cache is stored in contextvars.ContextVar variable, learning
our lessons from issue gh-11.

get_device_cached_queue(dev : dpctl.SyclDevice) -> dpctl.SyclQueue
get_device_cached_queue(
     (ctx: dpctl.SyclContext, dev : dpctl.SyclDevice)
) -> dpctl.SyclQueue

Function retrieves the queue from cache, or adds the new queue instance
there if previously absent.

Author: oleksandr-pavlyk

dpctl/_sycl_queue_manager.pxd

@@ -17,9 +17,12 @@
 # distutils: language = c++
 # cython: language_level=3
 
+from ._sycl_device cimport SyclDevice
 from ._sycl_queue cimport SyclQueue
 
 
 cpdef SyclQueue get_current_queue()
 cpdef get_current_device_type ()
 cpdef get_current_backend()
+
+cpdef object get_device_cached_queue(object)

dpctl/_sycl_queue_manager.pyx

@@ -20,6 +20,7 @@
 
 import logging
 from contextlib import ExitStack, contextmanager
+from contextvars import ContextVar
 
 from .enum_types import backend_type, device_type
 
@@ -35,6 +36,7 @@ from ._backend cimport (  # noqa: E211
     _device_type,
 )
 from ._sycl_context cimport SyclContext
+from ._sycl_device cimport SyclDevice
 
 __all__ = [
     "device_context",
@@ -44,6 +46,7 @@ __all__ = [
     "get_num_activated_queues",
     "is_in_device_context",
     "set_global_queue",
+    "_global_device_queue_cache",
 ]
 
 _logger = logging.getLogger(__name__)
@@ -291,3 +294,45 @@ def device_context(arg):
             _mgr._remove_current_queue()
         else:
             _logger.debug("No queue was created so nothing to do")
+
+
+cdef class _DeviceDefaultQueueCache:
+    cdef dict __device_queue_map__
+
+    def __cinit__(self):
+        self.__device_queue_map__ = dict()
+
+    def get_or_create(self, key):
+        """Return instance of SyclQueue and indicator if cache has been modified"""
+        if isinstance(key, tuple) and len(key) == 2 and isinstance(key[0], SyclContext) and isinstance(key[1], SyclDevice):
+            ctx_dev = key
+            q = None
+        elif isinstance(key, SyclDevice):
+            q = SyclQueue(key)
+            ctx_dev = q.sycl_context, key
+        else:
+            raise TypeError
+        if ctx_dev in self.__device_queue_map__:
+            return self.__device_queue_map__[ctx_dev], False
+        if q is None: q = SyclQueue(*ctx_dev)
+        self.__device_queue_map__[ctx_dev] = q
+        return q, True
+
+    cdef _update_map(self, dev_queue_map):
+        self.__device_queue_map__.update(dev_queue_map)
+
+    def __copy__(self):
+        cdef _DeviceDefaultQueueCache _copy = _DeviceDefaultQueueCache.__new__(_DeviceDefaultQueueCache)
+        _copy._update_map(self.__device_queue_map__)
+        return _copy
+
+
+_global_device_queue_cache = ContextVar('global_device_queue_cache', default=_DeviceDefaultQueueCache())
+
+
+cpdef object get_device_cached_queue(object key):
+    """Get cached queue associated with given device"""
+    _cache = _global_device_queue_cache.get()
+    q_, changed_ = _cache.get_or_create(key)
+    if changed_: _global_device_queue_cache.set(_cache)
+    return q_