Provide positional embedding on the hidden grid for transformer

[sahahner]

At the moment the latitude and longitude coordinates of the grid points are provided to the encoder only.

This PR implements the positional embedding for that transformer in the processor.

Related to this issue: https://github.com/ecmwf/aifs-dev/issues/10

Open points

• shape of positional embedding flexible
• make use of positional embedding more flexible. use hydra instantiation
  • Lat, Cos(lon), Sin(lon)
  • Cos(lat), Sin(lat), Cos(lon), Sin(lon)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (0deb66b) to head (59a5445).
Report is 2 commits behind head on develop.

Additional details and impacted files

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##           develop      #96   +/-   ##
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  Coverage    99.85%   99.85%           
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  Files           23       23           
  Lines         1374     1374           
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  Hits          1372     1372           
  Misses           2        2           

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[sahahner]

The latest commits make the positional encoding configurable and allow us to point to different ways to calculate the positional encodings given the latlons of the hidden grid.
I implemented the following two approaches:

• Lat, Cos(lon), Sin(lon)
  

• Cos(lat), Sin(lat), Cos(lon), Sin(lon)