Tutorials/Docs are break due to frontend API change

[antinucleon]

Att.. API is changed in https://github.com/dmlc/tvm/pull/3353/files
However doc and tutorials/docs such as AutoTVM are not updated.

Also I suggest to add a warning of this change, as it breaks all old code.

[tqchen]

The RFC was #3346 I agree it would be good to raise a bit more attention. @antinucleon can you point out which of the files that haven't been updated?

[zhiics]

#3401

@antinucleon Are you aware of any other places might miss the change?

[antinucleon]

Can we add a deprecated warning in frontend for 0.6 and remove in 0.7?

[tqchen]

Due to the fact that this is a return value, it is harder to deprecate the frontend API this time. Given that this is a major change that is necessary to enable more models(with loops recursion). I suggest we keep the change, but create another RFC notification thread in the forum.