Remove protocol="ucx" support in favor of distributed-ucxx (#9105)

* Remove UCX comm and tests

* Raise deprecation exception

* Improve warning/error message

Author: pentschev

distributed/comm/__init__.py

@@ -23,9 +23,14 @@ def _register_transports():
     backends["tls"] = tcp.TLSBackend()
 
     try:
-        from distributed.comm import ucx
+        # If `distributed-ucxx` is installed, it takes over the protocol="ucx" support
+        import distributed_ucxx
     except ImportError:
-        pass
+        try:
+            # Else protocol="ucx" will raise a deprecation warning and exception
+            from distributed.comm import ucx
+        except ImportError:
+            pass
 
 
 _register_transports()

distributed/comm/tests/test_comms.py

@@ -570,16 +570,6 @@ async def client_communicate(key, delay=0):
     listener.stop()
 
 
-@pytest.mark.gpu
-@gen_test()
-async def test_ucx_client_server(ucx_loop):
-    pytest.importorskip("distributed.comm.ucx")
-    ucp = pytest.importorskip("ucp")
-
-    addr = ucp.get_address()
-    await check_client_server("ucx://" + addr)
-
-
 def tcp_eq(expected_host, expected_port=None):
     def checker(loc):
         host, port = parse_host_port(loc)
@@ -1395,7 +1385,6 @@ async def test_do_not_share_buffers(tcp, list_cls):
     See Also
     --------
     test_share_buffer_with_header
-    test_ucx.py::test_do_not_share_buffers
     """
     np = pytest.importorskip("numpy")