Different behavior in DistilBERT when using "inputs_embeds"
Fixes #21089

# What does this PR do?
The behavior of the DistillBert model is wrong when the input embeddings are provided, and does not align with Bert or other models. 
This is due to the `Embeddings` layer that computes internally the addition of the positional embedding and word embedding. If the input embeddings are passed, they are not additioned with the positional embedding internaly. 


_The documentation is not available anymore as the PR was closed or merged._<|||||>cc @VictorSanh in case there was a specific reason behind this? <|||||>changes look good to me, thanks Arthur!

no specific reason behind it. my vague recollection is that `input_embeds` didn't exist when i first implemented it, so i would wait for core maintainer (or whatever the process is for transformers) to validate!