Sphinx intersphinx Inventory Support

[fangjunzhou]

I'm using sphinx for my own project and it seems that there's no inventory file on Taichi documentation website.

For most project using Sphinx, a objects.inv file should be generated to map internal references like https://numpy.org/doc/stable/objects.inv

This file is required for sphinx intersphinx extension to work as documented in https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html

For Taichi, the inventory file should be made available at https://docs.taichi-lang.org/objects.inv.

I believe Taichi is also using sphinx to auto generate api documentation. I wonder if the objects.inv was missed before deployment.