examples: Switch to adjoint time derivative

[mloubout]

Switch time derivative positon for the gradient as discussed on slack

tests pas locally so should be green.

[codecov]

Codecov Report

Merging #1706 (7521637) into master (87ee495) will decrease coverage by 0.22%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1706      +/-   ##
==========================================
- Coverage   88.85%   88.62%   -0.23%     
==========================================
  Files         208      208              
  Lines       33440    33440              
  Branches     4352     4352              
==========================================
- Hits        29713    29637      -76     
- Misses       3235     3311      +76     
  Partials      492      492              

Impacted Files	Coverage Δ
examples/seismic/acoustic/operators.py	96.15% <50.00%> (ø)
examples/seismic/tti/operators.py	97.09% <100.00%> (ø)
tests/test_gpu_openacc.py	56.69% <0.00%> (-40.95%)	⬇️
devito/passes/iet/languages/openacc.py	77.01% <0.00%> (-14.95%)	⬇️
devito/arch/compiler.py	54.95% <0.00%> (-3.20%)	⬇️
devito/types/array.py	93.15% <0.00%> (-0.69%)	⬇️
devito/ir/iet/visitors.py	80.52% <0.00%> (-0.38%)	⬇️
devito/passes/equations/linearity.py	100.00% <0.00%> (+1.72%)	⬆️

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[FabioLuporini]

Since this has implications on GPU+data streaming, we should wait for #1702 to land first, which also "fixes" the current mismatching tag issue that is causing the omp offloading job to get scheduled to wampa rather than kimogila

[mloubout]

sure

[FabioLuporini]

Merged, thanks