[Relay] Fix reduce axis bug

[altanh]

Fixes failure of sum and all on axis=0. Added regression tests as well.

[altanh]

cc @siju-samuel @jroesch

[altanh]

It looks like this change was undone in the past for #2439, but I can't see how axis and isinstance(axis, int) differs from simply isinstance(axis, int) other than evaluating to false when axis=0, since isinstance(None, int) == False.

[jroesch]

We need to prevent against the axis being None, it should be axis is not None and ... what happens if you pass None right now?

[altanh]

Not quite sure I understand, unless the existing code was incorrect in keeping None:

>>> axis = None
>>> axis = [axis] if axis and isinstance(axis, int) else axis
>>> axis is None
True

For clarity, this demonstrates the bug this PR fixes:

>>> axis = 0
>>> axis = [axis] if axis and isinstance(axis, int) else axis
>>> axis == [0]
False

whereas with the fix:

>>> axis = 0
>>> axis = [axis] if isinstance(axis, int) else axis
>>> axis == [0]
True
>>> axis = None
>>> axis = [axis] if isinstance(axis, int) else axis
>>> axis is None
True

Otherwise TVM crashes since it expects an array of ints or None, from what I can tell.

[tqchen]

THanks, @altanh @jroesch, this PR is now merged