Compatible changes for v1.1.0 rc

[ailzhang]

We're in the process of getting v1.1.0 release rc wheel and noticed this
PR is required for stannum to work v1.1.0.

v1.1.0 tracking: https://github.com/taichi-dev/taichi/milestone/5

[ifsheldon]

I added a conditional import to not break the import when the users use Taichi < 1.1.0

BTW, can you link the corresponding Taichi issue?

[ailzhang]

@ifsheldon We track v1.1.0 release in a milestone so I put it in the description as well.

Btw this change is not included in our release note in the sense that we don't expect taichi users to directly use classes from taichi._lib.core.taichi_python. For those public facing apis we currently maintain that in https://github.com/taichi-dev/taichi/blob/master/tests/python/test_api.py. But we do understand that as a library built on top of taichi, stannum actually depends on some taichi internals so we may plan to collect and properly expose those classes/types/utils as well. Wdyt?

[ifsheldon]

Yeah, I think so. I think at least Taichi types (scalar, vector, field types and ndarray and kernel types) can be public.