Fix lint issues.

tensorflow_federated/python/core/impl/executor_stacks.py

...orflow_federated/python/core/impl/set_default_executor.py

tensorflow_federated/python/core/impl/type_utils.py

...flow_federated/python/core/impl/utils/tensorflow_utils.py

...ted/python/research/baselines/emnist/run_dp_experiment.py

...derated/python/research/baselines/emnist/run_federated.py

tensorflow_federated/python/research/utils/training_loops.py

...ow_federated/python/research/utils/training_loops_test.py

...orflow_federated/python/research/utils/utils_impl_test.py