[relay][pass manager] Open transform namespace

[zhiics]

This PR is the first item of #3202

Now we open the transform namespace by adding transform.h and transform.py. Currently, pass manager related code is moved to these two files. The ir_pass related code is left in the original place. They will be moved once we finish porting.

cc @jroesch @tqchen @MarisaKirisame @wweic @slyubomirsky

[zhiics]

I am also going to change sequential_pass and SequentialPass to sequential. That is what we intended, right? @tqchen @MarisaKirisame

[tqchen]

We don’t want Sequential and sequential, both of which are duplicated.

Consider merge as Sequential(use constructor in py and possibly the same function name in cxx).

Let us double check the desirable API. Specially, we want to have something like

Pass FoldConstant();

[tqchen]

A quick way to check is to see if we can reproduce the example shown in the RFC

[zhiics]

I think we should be able to execute that example using the current Sequential, like https://github.com/dmlc/tvm/blob/master/tests/python/relay/test_pass_manager.py#L357

The problem is that we need to finish pass registration, meaning we need to have Pass FoldConstant() first. This is the goal of #3191, @MarisaKirisame is working on it.

But I currently haven't used BuildConfig in Sequential pass yet.

[tqchen]

declare node, but hide the implementation inside. (User don't need to see SequentialNode, they only need to see Sequantial)

[tqchen]

Merge this in for now. Thanks, @zhiics. I will propose a few improves in a follow-up PR