Merge pull request #1975 from NNPDF/fix-multidense-tf216

.github/workflows/python_installation.yml

n3fit/src/n3fit/backends/keras_backend/MetaLayer.py

n3fit/src/n3fit/backends/keras_backend/MetaModel.py

n3fit/src/n3fit/backends/keras_backend/callbacks.py

n3fit/src/n3fit/backends/keras_backend/constraints.py

n3fit/src/n3fit/backends/keras_backend/multi_dense.py

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

n3fit/src/n3fit/hyper_optimization/hyper_scan.py

n3fit/src/n3fit/layers/observable.py

n3fit/src/n3fit/model_gen.py

n3fit/src/n3fit/stopping.py

n3fit/src/n3fit/tests/test_hyperopt.py

n3fit/src/n3fit/tests/test_multidense.py