Update RMM allocators

Following rapidsai/rmm#1221, RMM allocators now
live under the `rmm.allocators` submodule and using old paths causes
warnings to be raised. This change updates to the new submodule and
prevents such warnings.

python/cudf/cudf/__init__.py

@@ -8,6 +8,8 @@
 from numba import config as numba_config, cuda
 
 import rmm
+from rmm.allocators.cupy import rmm_cupy_allocator
+from rmm.allocators.numba import RMMNumbaManager
 
 from cudf import api, core, datasets, testing
 from cudf.api.extensions import (
@@ -96,8 +98,8 @@
 
     del patch_numba_linker_if_needed
 
-cuda.set_memory_manager(rmm.RMMNumbaManager)
-cupy.cuda.set_allocator(rmm.rmm_cupy_allocator)
+cuda.set_memory_manager(RMMNumbaManager)
+cupy.cuda.set_allocator(rmm_cupy_allocator)
 
 try:
     # Numba 0.54: Disable low occupancy warnings