Merge pull request #1788 from NNPDF/trvl-mask-layers

extra_tests/regression_fits/central_16.exportgrid

extra_tests/regression_fits/central_16.json

extra_tests/regression_fits/diagonal_45.exportgrid

extra_tests/regression_fits/diagonal_45.json

extra_tests/regression_fits/feature_scaling_81.json

extra_tests/regression_fits/flavour_29.exportgrid

extra_tests/regression_fits/flavour_29.json

extra_tests/regression_fits/no_lagrange_27.exportgrid

extra_tests/regression_fits/no_lagrange_27.json

extra_tests/regression_fits/no_msr_92.exportgrid

extra_tests/regression_fits/no_msr_92.json

extra_tests/regression_fits/no_sumrules_18.exportgrid

extra_tests/regression_fits/no_sumrules_18.json

extra_tests/regression_fits/no_vsr_54.exportgrid

extra_tests/regression_fits/no_vsr_54.json

extra_tests/regression_fits/normal_fit_72.exportgrid

extra_tests/regression_fits/normal_fit_72.json

extra_tests/regression_fits/trainable_prepro_61.exportgrid

extra_tests/regression_fits/trainable_prepro_61.json

n3fit/src/n3fit/backends/keras_backend/operations.py