GPU version of diffusion-PDE-based smoothing

[tianshi-liu]

GPU version of #1725.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 121 lines in your changes missing coverage. Please review.

Project coverage is 45.46%. Comparing base (f8696dd) to head (2125741).
Report is 2 commits behind head on devel.

Files with missing lines	Patch %	Lines
...postprocess_sensitivity_kernels/smooth_sem_pde.F90	0.00%	101 Missing ⚠️
src/gpu/specfem3D_gpu_cuda_method_stubs.c	0.00%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #1819      +/-   ##
==========================================
+ Coverage   44.38%   45.46%   +1.08%     
==========================================
  Files         273      273              
  Lines       64694    64785      +91     
==========================================
+ Hits        28712    29454     +742     
+ Misses      35982    35331     -651     

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