Fix typos by replacing `TrainableModel` `Model`.

docs/federated_learning.md

...ted/python/research/adaptive_lr_decay/adaptive_fed_avg.py

tensorflow_federated/python/research/flars/flars_fedavg.py

...d/python/research/optimization/shared/fed_avg_schedule.py

...search/robust_aggregation/robust_federated_aggregation.py

...h/robust_aggregation/robust_federated_aggregation_test.py

...erated/python/research/targeted_attack/attacked_fedavg.py