DeviceBuffer: accept memory resource when taking ownership

In c_from_unique_ptr we should not just rely on
get_current_device_resource, but rather allow the user to pass in the
memory resource they _know_ was used to allocate the buffer we are
taking ownership of.

So that we are backwards-compatible we default, as before, to the
current device resource.

python/rmm/rmm/_lib/device_buffer.pxd

@@ -65,7 +65,8 @@ cdef class DeviceBuffer:
     @staticmethod
     cdef DeviceBuffer c_from_unique_ptr(
         unique_ptr[device_buffer] ptr,
-        Stream stream=*
+        Stream stream=*,
+        DeviceMemoryResource mr=*,
     )
 
     @staticmethod

python/rmm/rmm/_lib/device_buffer.pyx

@@ -33,6 +33,7 @@ from cuda.ccudart cimport (
 )
 
 from rmm._lib.memory_resource cimport (
+    DeviceMemoryResource,
     device_memory_resource,
     get_current_device_resource,
 )
@@ -48,7 +49,8 @@ cdef class DeviceBuffer:
     def __cinit__(self, *,
                   uintptr_t ptr=0,
                   size_t size=0,
-                  Stream stream=DEFAULT_STREAM):
+                  Stream stream=DEFAULT_STREAM,
+                  DeviceMemoryResource mr=None):
         """Construct a ``DeviceBuffer`` with optional size and data pointer
 
         Parameters
@@ -65,6 +67,9 @@ cdef class DeviceBuffer:
             scope while the DeviceBuffer is in use. Destroying the
             underlying stream while the DeviceBuffer is in use will
             result in undefined behavior.
+        mr : optional
+           DeviceMemoryResource for the allocation, if not provided
+           defaults to the current device resource.
 
         Note
         ----
@@ -80,7 +85,7 @@ cdef class DeviceBuffer:
         cdef const void* c_ptr
         cdef device_memory_resource * mr_ptr
         # Save a reference to the MR and stream used for allocation
-        self.mr = get_current_device_resource()
+        self.mr = get_current_device_resource() if mr is None else mr
         self.stream = stream
 
         mr_ptr = self.mr.get_mr()
@@ -162,13 +167,14 @@ cdef class DeviceBuffer:
     @staticmethod
     cdef DeviceBuffer c_from_unique_ptr(
         unique_ptr[device_buffer] ptr,
-        Stream stream=DEFAULT_STREAM
+        Stream stream=DEFAULT_STREAM,
+        DeviceMemoryResource mr=None,
     ):
         cdef DeviceBuffer buf = DeviceBuffer.__new__(DeviceBuffer)
         if stream.c_is_default():
             stream.c_synchronize()
         buf.c_obj = move(ptr)
-        buf.mr = get_current_device_resource()
+        buf.mr = get_current_device_resource() if mr is None else mr
         buf.stream = stream
         return buf