ENH: support cudf Buffer BufferOwner structure (#798)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

python/xorbits/_mars/_utils.pyx

@@ -286,7 +286,7 @@ def tokenize_cupy(ob):
 def tokenize_cudf(ob):
  from xoscar.serialization import serialize
  header, buffers = serialize(ob)
- return iterative_tokenize([header] + [(buf.ptr, buf.size) for buf in buffers])
+ return iterative_tokenize([header] + [(buf._owner._ptr, buf.size) for buf in buffers])
 
 
 cdef Tokenizer tokenize_handler = Tokenizer()