[deepspeed] deal with models w/o `config.hidden_size`
This PR 
1. extends the deepspeed integration config setup to models that use `config.hidden_sizes` instead of `config.hidden_size` 
2. and while at it adds a check so that it only ever tries to figure hidden size if it's actually needed. 
3. and if the model config has neither of the 2 attributes - a proper assert is used.

Fixes: https://github.com/huggingface/transformers/issues/21342
_The documentation is not available anymore as the PR was closed or merged._<|||||>cc: @pacman100 - you may probably want to sync this special case with accelerate, right?