Correct num_device value in initialize_GPU

[eyalshimony]

Change the assignment of num_device in cases where NPROC > 1 and NUMBER_OF_SIMULTANEOUS_RUNS > 1 to prevent assignment of multiple processes to the same GPU

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 49.75%. Comparing base (8f39006) to head (b4a8a95).
Report is 2 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/specfem3D/initialize_simulation.F90	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #1783      +/-   ##
==========================================
- Coverage   49.76%   49.75%   -0.01%     
==========================================
  Files         272      272              
  Lines       62908    62910       +2     
==========================================
  Hits        31304    31304              
- Misses      31604    31606       +2     

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[danielpeter]

addresses issue #1782