Support None type

[take-cheeze]

Currently they are treated as False in ch2o:

chainer-compiler/ch2o/ch2o/utils.py

Line 112 in 2298109

# TODO(hamaji): Revisit to check if this decision is OK.

How to check:

diff --git a/ch2o/tests/syntax/Cmp.py b/ch2o/tests/syntax/Cmp.py
index fa8913b..e1ab3fc 100644
--- a/ch2o/tests/syntax/Cmp.py
+++ b/ch2o/tests/syntax/Cmp.py
@@ -67,6 +67,7 @@ if __name__ == '__main__':
         for x, y in [(None, None),
                      (42, None),
                      (True, None),
+                     (False, None),
                      (True, False),
                      (True, True),
                      (False, False),
@@ -75,4 +76,3 @@ if __name__ == '__main__':
                      (np.array(42), np.array(42))]:
             ch2o.generate_testcase(cls(), [x, y],
                                    subname='%s_%s_%s' % (x, name, y))
-

$ python3 scripts/ch2ocheck.py ch2o/tests/syntax/Cmp.py

Thoughts to implement:

• Rename current Null kind of ChxVMVar to Undefined
• Mark a special opaque kind with name None in ONNX representation