[Phi2] Add support for phi2 models
Author: susnato
Pull request: yes

# What does this PR do?

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This PR fixes the conversion script for `phi` to include the changes made in this [commit](https://huggingface.co/microsoft/phi-1/commit/bbace889388b8bb0ba6ec3d28dcccca00f962062). It also adds a integration test for `phi2`. 


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## Who can review?

@gugarosa @ArthurZucker 

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Models:

- text models: @ArthurZucker and @younesbelkada
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- flax: @sanchit-gandhi
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Comments:
> Hey! Thanks for the PR, alright overall, but let's not add the phi2 changes here since it's not supported!

Hi @ArthurZucker, phi2 has the same modeling code as the library phi model...so I guess it's supported. 
I also created [susnato/phi-2](https://huggingface.co/susnato/phi-2) to verify the outputs and they are ok.

I am sorry if I am missing something here...

Oh no I checked #28163 and thought we needed a lot more changes! If this is enough let's rename this one to `[Phi2] Add support for phi2 models` and close the other one? 
(We agree that this is the only changes needed to support phi2 ?)


Hi @ArthurZucker, yes this is enough for support of `phi2`(weights conversion script and verify logits in test) but #28163 is for adding more features for both phi1 and phi2 models so we should not close that IMO. 

Alright! Got it! 

Hi @ArthurZucker, I have pushed the changes.  

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_28211). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Btw it’s mostly nits we can merge without them  as the community is pretty eager to have it!🤗

Hi @ArthurZucker, sorry I forgot to update the doc file to include the `phi2` example, I guess we should add an example and show it because many people were unable to use it with the library and raised issues in `microsoft/phi-2` (until the weights are in appropriate order we should use `susnato/phi-2` instead of the official repo to load it successfully). If you agree, I would create a quick PR to add the example. WDYT? 

Should be alright,. we'll do a release this week as well so that everything will be included and the weights will have to be changed as well! cc @gugarosa for visibility