Generate: add test for left-padding support
# What does this PR do?

This PR adds a test to check whether a decoder-only model supports left padding. The test was somewhat tricky to design -- the reasoning is all commented in the code, for future reference.

Let me know if you agree with the decisions made in the test!

Hopefully we will be able to detect whether a model supports left-padding before merging 🤞 
Current failures -- including Llama and GPTNeoX, the two I had my eyes on:
![Screenshot 2023-03-22 at 18 42 57](https://user-images.githubusercontent.com/12240844/227005881-868067e5-a083-4e87-9858-020694869843.png)
<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger reduced to 10 runs, merging when CI gets green :)