[tutorial] Relay pass infra tutorial

[zhiics]

Add a tutorial for the pass infra.

[billzorn]

Is there a tutorial for adding a new custom pass, completely in Python?

I think it would be useful to have some general, easy to find documentation on "working with the Relay IR / AST in Python." There are applications beyond just optimizing programs - I found while writing a tool to convert Relay to FPCore that working with the IR in Python was actually very convenient, but I had to have a Relay expert on hand to explain how to do it.

EDIT: More specifically, I had to have a Relay expert on hand to convince me that I could work with the IR in Python - after looking at the existing documentation, I had convinced myself that the only way to interact with it was to write c++.

[zhiics]

@billzorn Sorry for the late response. I've been quite busy in the past several weeks. I will try to find some cycles next week to finish a version for this tutorial so that we can start reviewing.

I've covered an example in this tutorial about writing a customized pass in Python. You can probably take a look. Any feedback is highly appreciated.

[zhiics]

@weberlo @slyubomirsky @billzorn @tqchen @wweic @yongwww @jroesch please review when you have time. Thanks.

[yzhliu]

Thanks @zhiics @billzorn @MarisaKirisame @yongwww