XGLM: Fix left-padding (PT and TF)
# What does this PR do?

Fixes left-padding for XGLM, on PT and TF. It is the usual problem: `position_ids` was not being used/passed around, and the code assumed that position ids = past length + input length (which is not true when left-padding is present).

Fixes #22707 

While touching XGLM, other issues were sorted:
1. on PT, docs were duplicated (and one of the copies was wrong)
2. XGLM generate with sampling integration test was pinned on CPU (as always, GPU gives different results, which was making our slow CI report an error)
3. TF XLA was failing because of this (left padding support), so now we have TF XLA XGLM 🙌 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@amyeroberts now it is ready :)<|||||>Adding a new keyword argument is not considered breaking, so that's fine!