Properly enable Cython docstrings. (#1020)

This PR embeds signatures in Cython functions and classes so that they'll be accessible for normal Python help. While docstrings are currently present as expected, we still have to provide Cython with the appropriate directives to tell it that we want signatures.

Authors:
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Ashwin Srinath (https://github.com/shwina)
  - Bradley Dice (https://github.com/bdice)

URL: #1020

python/CMakeLists.txt

@@ -14,9 +14,12 @@
 
 cmake_minimum_required(VERSION 3.20.1 FATAL_ERROR)
 
-# TODO: Pull this directly from the file written by rapids_cmake_write_version_file.
 set(rmm_version 22.06.00)
 
+set(CYTHON_FLAGS
+    "--directive binding=True,embedsignature=True,always_allow_keywords=True"
+    CACHE STRING "The directives for Cython compilation.")
+
 project(
   rmm-python
   VERSION ${rmm_version}