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

Adding Time Series Autoformer model https://arxiv.org/abs/2106.13008

Related issue: #21890 

@kashif :) 

## Differences between the vanilla transformer

<img width="484" alt="image" src="https://user-images.githubusercontent.com/17675462/229438415-6a5bba78-c3bf-47b1-966a-f3664a0921e0.png">

* Introduced Series Decomposition in encoder & decoder --- done, waiting for review
* Replaced canonical self-attention with autocorrelation block --- done, waiting for review
* Added seasonal and trend inputs for the decoder --- added todo places in the code
* trend seasonal pseudo code:
```
mean_data = mean(enc_input)
zeros = zeros() # size: x_dec.size(0), prediction_length, x_dec.size(2)
seasonal_init, trend_init = decomp_layer(enc_input)

trend_init = concat(trend_init, mean_data)
seasonal_init = concat(seasonal_init, zeros)
```

One small open issue left, is adding the series decomposition to the decoder with the trend input. Will do after the initial review :)<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>some of the TF tests are failing and I believe they are unrelated<|||||>PR is green <|||||>thank you @amyeroberts will get it fixed!<|||||>@amyeroberts, thank you for the comprehensive CR! I sincerely appreciate the effort and time you dedicated to thoroughly assessing this pull request. 

Will be fixed!<|||||>CR changes I did:


* Added `layer_norm_eps` 
* Model layers are now take the config, except the `AutoformerAttention` which I wasn't sure about
* Better variables names 
* Addressed questions I could answer

`fix-copies` is falling because diffs with the time-series-transformer. It's about to decide if to change time-series-transformer here, or to remove "copied from..."

@kashif @amyeroberts 

