Features/1243 refactoring of communication separate mpi4py wrappers from dn darrays

[mrfh92]

This PR addresses #1243

Description: TBD ... see comments below

[mrfh92]

Current problem: everything fails due to circular imports

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[mrfh92]

Done so far:

• a new module communication_backends has been introduced
• communication.py from heat/core has been divided into communications.py (containing the basis Communication-class) and mpi4py4torch.py (containing the MPICommunication-class and the wrappers for mpi4py-functionality for torch)
• calling any MPI-communication with DNDarrays as buffers is deprecated; everywhere in the code such calls have been replaced by the equivalent way using the local arrays of a DNDarray as buffers instead (actually, such a check and reference of the local arrays has been done in all communication-functions previously)

To do:

• the tests for the no-more existing heat/core/communication.py are still in heat/core/tests/test_communication.py (although being adapted according to the last bullet point above); however, it would be favourable to rewrite them as tests to heat/communication_backends/mpi4py4torch.py, i.e. to remove any reference to DNDarrays there in order to achieve a clear separation.

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Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (6a5115c) 91.81% compared to head (47a4d25) 91.83%.
Report is 1 commits behind head on main.

❗ Current head 47a4d25 differs from pull request most recent head abe3d20. Consider uploading reports for the commit abe3d20 to get more accurate results

Files	Patch %	Lines
heat/core/linalg/solver.py	50.00%	4 Missing ⚠️
heat/communication_backends/communication.py	96.36%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1265      +/-   ##
==========================================
+ Coverage   91.81%   91.83%   +0.01%     
==========================================
  Files          79       79              
  Lines       11463    11171     -292     
==========================================
- Hits        10525    10259     -266     
+ Misses        938      912      -26     

Flag	Coverage Δ
unit	91.83% <94.59%> (+0.01%)	⬆️

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Thank you for the PR!