message: Updated the equation as per the paper
  Used this to update the paper.
diff: @@ -47,9 +47,10 @@ class Ftrl(optimizer_v2.OptimizerV2):
  $$n_{t,i} = n_{t-1,i} + g_{t,i}^{2}$$
  $$\sigma_{t,i} = (\sqrt{n_{t,i}} - \sqrt{n_{t-1,i}}) / \alpha$$
  $$z_{t,i} = z_{t-1,i} + g_{t,i} - \sigma_{t,i} * w_{t,i}$$
  - $$w_{t,i} = - ((\beta+\sqrt{n_{t,i}}) / \alpha + 2 * \lambda_{2})^{-1} *
  - (z_{i} - sgn(z_{i}) * \lambda_{1}) if \abs{z_{i}} > \lambda_{i}
  - else 0$$
  + $$w_{t,i} =\begin{cases}
  +0 & | z_{i}| \leqslant \lambda _{1} ,\\
  +\ -\left(\frac{\beta +\sqrt{n_{t,i}}}{\alpha } +\lambda _{2}\right)^{-1}( z_{i} -sgn(z_{i} )*\lambda _{1}) & otherwise.
  +\end{cases}$$
  Check the documentation for the l2_shrinkage_regularization_strength
  parameter for more details when shrinkage is enabled, in which case gradient
  