Apollo Optimizer in Tensorflow 2.x

Notes:

• Warmup is important with Apollo optimizer, so be sure to pass in a learning rate schedule vs. a constant learning rate for learning_rate. One cycle scheduler is given as an example in one_cycle_lr_schedule.py
• To clip gradient norms as in paper, add either clipnorm (parameter-wise clipping by norm) or global_clipnorm to the arguments (for example clipnorm=0.1).
• Decoupled weight decay is used by default.