[Time-Series] informer model
# What does this PR do?

Adding Time Series Informer model https://arxiv.org/abs/2012.07436

Related issue: #20903 

@kashif :) 
very cool! Having a look shortly!<|||||>> very cool! Having a look shortly!

Wow you saw it fast! right now it's just the template of the vanilla TS transformer. 

BTW I sent you an email :)<|||||>Hi, @NielsRogge  and @kashif 🙂

Maybe you have an example for a conversion script?
I'm following the [How to add a model to 🤗 Transformers?](https://huggingface.co/docs/transformers/add_new_model), section six "Write a conversion script":

> Don’t hesitate to ask the Hugging Face team to point you to a similar already existing conversion script for your model.

Thank you so much,
Eli<|||||>thanks! having a look!<|||||>> thanks! having a look!

Work is still in progress, but you might have a look if you have time :) 
And by the way, your implemention and the vanilla TS are helping me a lot!<|||||>Hi @kashif, I fixed the final attention output of ProbSparseAttention,  and added the ProbMask. In more detail:

# Major
1. Added calculation of the final `attn_output` using `v_aggregated`, meaning steps 7 & 8 in the following:

<img width="446" alt="image" src="https://user-images.githubusercontent.com/17675462/218316971-ce4dbe9e-d677-48e6-9184-0043ca179e6a.png">

**Reference:** [Informer paper](https://arxiv.org/abs/2012.07436), **Section:** "Implement of the ProbSparse self-attention"

2. Added ProbMask, function name `_prepare_decoder_prob_attention_mask`.


# Minor 
1. Comment-in attention dropout in `ProbSparseAttention` since the original impl didn't use it.
2. Removed `attention_mask` for the encoder, since the original impl don't apply it to the encoder, only for the decoder.
3. Added `self.attn = config.attn` in the decoder, to later check if to create ProbMask or the standard casual mask.
4. Removed unused code from the original impl.

Now the tests are falling mostly because assertion errors. Before continuing to fix them, I would appreciate if you can have a look :)

Thanks,
Eli<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>thanks @sgugger will get it fixed!<|||||>@kashif fixed what I could from Sylvain comments.
The main thing is that some tests are breaking after this fix https://github.com/huggingface/transformers/pull/21099/commits/b4cbddfa05e3bd739b79569cd3c3b89e316f2451
<|||||>@sgugger I have fixed all the assert issues in the PR #21846 and will fix the copies here when that gets merged