[Refactor][Relay] Refactor Relay Python to use new FFI

[zhiics]

#5068

This PRs refactors Relay to

1. Reflect the C++ side organization to keep consistency
2. Separated analysis and transform namespace to make the namespace for different optimizations clear
3. Introduced the new _ffi_api for each namespace and remove the old _xx.py files
4. Removed some op related files to import modules from subdirectories and instead import then from init.py directly to keep Relay directory clean

Three followup PRs are needed to:

1. Create a function.py file for relay.Function
2. Move the files that are left over to the backend directory
3. Docs

cc @tqchen @jroesch @icemelon9 @slyubomirsky @MarisaKirisame @wweic @yzhliu

[tqchen]

Some comments about docs and structure exposure.

• We could consider remove register_relay_node and just call into tvm._ffi.register_object
• For user facing python APIs, we want to make sure that they are exported in a consistent way with the namespace structure.
  • For example, while a lot of APIs are defined in tvm.tir.expr, we only expose them in tvm.tir https://docs.tvm.ai/api/python/tir.html
  • The API generation is also greatly simplified https://github.com/apache/incubator-tvm/blob/master/docs/api/python/tir.rst
  • This means ideally we want to show tvm.relay.Constant in our docs.
  • Notably, we might encouter aliases, in which case we want to be able to mark them via https://github.com/apache/incubator-tvm/blob/master/docs/conf.py#L255, see https://docs.tvm.ai/api/python/te.html#tvm.te.abs

[tqchen]

In the case of the file groupings for relay/expr.py etc. There are two ways to think about it. We can either make them consistent with src/, which means we want relay/ir/expr.py, or first to be consistent with the include, in which case it is relay/expr.py.

Given that tir already tries to be consistent with include first(as it is user facing), let us try to also do that in relay

[zhiics]

@tqchen I updated the change. Moving the files back to the relay folder actually also resolves the user facing python API concern.

[tqchen]

Thanks @zhiics please also take note about the above comments for docs in general(see the new usage of automodule that removes the need of manual autofunction), but we can do that as a followup if CI is green

• https://github.com/apache/incubator-tvm/blob/master/docs/api/python/tir.rst
• https://github.com/apache/incubator-tvm/blob/master/docs/conf.py#L255

[tqchen]

some further comments

[zhiics]

@tqchen PTAL, thanks.

[tqchen]

Thanks @zhiics