SARL $1, (BX)                           // d13b
	SARL $1, (R11)                          // 41d13b
	SARL $1, DX                             // d1fa
	SARL $1, R11                            // 41d1fb
	SARL CL, (BX)                           // d33b
	SARL CL, (R11)                          // 41d33b
	SARL CL, DX                             // d3fa
	SARL CL, R11                            // 41d3fb

    an alternative to accepting a `callable` loss.
*   Adds `beta` hyperparameter to
    [FTRL](https://www.tensorflow.org/api_docs/python/tf/keras/optimizers/Ftrl)
    optimizer classes (Keras and others) to match
    [FTRL paper](https://research.google.com/pubs/archive/41159.pdf).
*   `Optimizer.__init__` now accepts a `gradient_aggregator` to allow for