BLOOM - Fix mask creation by default
- fix niche case where the mask is not fed to the forward function

cc @justheuristic 
_The documentation is not available anymore as the PR was closed or merged._<|||||>This happens if user decides to run `model(input_ids=some_tensor)`, which originally happened during prototyping (in a notebook).

It is indeed a niche case;
However, the fact that mask _is_ optional can hint other users to the idea that they can keep it None - and it would be more intuitive if having mask=None would be equivalent to passing all ones, like in GPT2Model for example.