[docs] Update relay docs

[zhiics]

#5068

This PR updates Relay docs. It removes redundant files and automatically generates automodules.

cc @tqchen @comaniac @cchung100m

[tqchen]

We can also consider allow these members, but modify https://github.com/apache/incubator-tvm/blob/master/docs/conf.py#L254 to mark them as alias

[zhiics]

Yeah, I tried this before. But there are a lot of places that directly use the classes without namespace. It causes many ambiguous reference. For example:

https://github.com/apache/incubator-tvm/blob/master/python/tvm/ir/adt.py#L82

Constructor here could be tvm.ir.Constructor or tvm.relay.Constructor

[zhiics]

@tqchen BTW, the CI still complains about the conflicts from relay_op.rst even I removed the autofunction there. I couldn't repro it locally. Is it due to some caching problem?

[tqchen]

interesting, can you try to use the exact same docker image? Iet us also try to add make clean before https://github.com/apache/incubator-tvm/blob/master/tests/scripts/task_sphinx_precheck.sh#L36

[zhiics]

I checked out the image and ran the same command before. It seemed working fine. Hopefully I didn't do something silly. I will double check.

[zhiics]

@tqchen Yeah, it should be cached before. make clean solves the problem. Thanks.

[tqchen]

Great, let us also add make clean to https://github.com/apache/incubator-tvm/blob/master/tests/scripts/task_python_docs.sh#L47