[generate] do_sample default back to False
This somewhat reverts the commit:
https://github.com/huggingface/transformers/commit/6c1b23554f8bb5b5e1f6c80969acab764c755678

and the decision taken in #2696

and sets the default sampling behavior of `generate()` to greedy - / beam search. 

Pros:
- `False` is the more natural default value 
- Prettier API (especially for encoder_decoder models which will mostly only use beam search generate()) 

Cons:
- Some people might aleady be used to the `do_sample=True` default value and this commit might break the logic of their code (but would be trivial to change for them) 

I'm somewhat indifferent whether this PR should be merged, but I think @thomwolf and @sshleifer  are in favor of it. 

@LysandreJik @thomwolf @sshleifer 
Not 100% sure how this results in a "Prettier API," but agree this isn't a big deal to fix downstream. (my current code explicitly sets `do_sample=True` just in case something like this happened.)

If you are creating any demo generation notebooks/tooling like Write With Transformer, I recommend explicitly noting this behavior.