Optimizer that implements the Adam algorithm.

Inherits From: `Optimizer`

### Used in the notebooks

Adam optimization is a stochastic gradient descent method that is based on adaptive estimation of first-order and second-order moments.

According to Kingma et al., 2014, the method is "computationally efficient, has little memory requirement, invariant to diagonal rescaling of gradients, and is well suited for problems that are large in terms of data/parameters".

`learning_rate` A `Tensor`, floating point value, or a schedule that is a `tf.keras.optimizers.schedules.LearningRateSchedule`, or a callable that takes no arguments and returns the actual value to use, The learning rate. Defaults to 0.001.
`beta_1` A float value or a constant float tensor, or a callable that takes no arguments and returns the actual value to use. The exponential decay rate for the 1st moment estimates. Defaults to 0.9.
`beta_2` A float value or a constant float tensor, or a callable that takes no arguments and returns the actual value to use, The exponential decay rate for the 2nd moment estimates. Defaults to 0.999.
`epsilon` A small constant for numerical stability. This epsilon is "epsilon hat" in the Kingma and Ba paper (in the formula just before Section 2.1), not the epsilon in Algorithm 1 of the paper. Defaults to 1e-7.
`amsgrad` Boolean. Whether to apply AMSGrad variant of this algorithm from the paper "On the Convergence of Adam and beyond". Defaults to `False`.
`name` Optional name for the operations created when applying gradients. Defaults to `"Adam"`.
`**kwargs` Keyword arguments. Allowed to be one of `"clipnorm"` or `"clipvalue"`. `"clipnorm"` (float) clips gradients by norm; `"clipvalue"` (float) clips gradients by value.

#### Usage:

````opt = tf.keras.optimizers.Adam(learning_rate=0.1)`
`var1 = tf.Variable(10.0)`
`loss = lambda: (var1 ** 2)/2.0       # d(loss)/d(var1) == var1`
`step_count = opt.minimize(loss, [var1]).numpy()`
`# The first step is `-learning_rate*sign(grad)``
`var1.numpy()`
`9.9`
```