Deberta v2 code simplification
# What does this PR do?

This PR simplifies and fixes the code for the DeBERTa V2 disentangled attention bias calculation:
1. Removes a spurious substraction of the type `x - x` always resulting in 0
2. Fixes condition checking for the attention type and attention score calculation. In the current version, the additional check to see if `p2p` is in the attention type is performed at the wrong position (https://github.com/huggingface/transformers/blob/2c2a31ffbcfe03339b1721348781aac4fc05bc5e/src/transformers/models/deberta_v2/modeling_deberta_v2.py#L781).  The `p2c` attention is not used for `p2p`, but `c2p` is. Currently the execution would fail if the attention types include `p2p` but not `c2p`: the c2p_pos variable used https://github.com/huggingface/transformers/blob/2c2a31ffbcfe03339b1721348781aac4fc05bc5e/src/transformers/models/deberta_v2/modeling_deberta_v2.py#L813 would not be defined at https://github.com/huggingface/transformers/blob/2c2a31ffbcfe03339b1721348781aac4fc05bc5e/src/transformers/models/deberta_v2/modeling_deberta_v2.py#L772. This PR moves the check for the `p2p` attention flag to the right position and simplifies the `p2c` attention calculation.

## Who can review?

@LysandreJik
@BigBird01 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15732). All of your documentation changes will be reflected on that endpoint.<|||||>Hello @LysandreJik , @BigBird01 ,
As discussed a couple of weeks back I wanted to reach out to see if any further work would be required on this PR.

Thank you!<|||||>LGTM, also pinging @anton-l  as it affects SEW/SEW-D<|||||>Hello @LysandreJik , @anton-l 
I was wondering if there is further work required on this PR?
Thank you!<|||||>Hi @guillaume-be, no further changes required from my side (SEW-D models), feel free to merge if everything else is ok 🙂 <|||||>Thanks for your PR, merging!