message: Polish the tf.optimizers.Adagrad class documentation a bit.
  Before:
diff: @@ -41,18 +41,18 @@ class Adagrad(optimizer_v2.OptimizerV2):
  the smaller the updates.
  Initialization:
  + $$accum_{g_0} := \text{initial_accumulator_value}$$
  - $$accum_g_0 := initial_accumulator_value$$
  -
  + Update step:
  $$t := t + 1$$
  - $$accum_g_t := accum_g_{t-1} + g * g$$
  - $$theta_t := theta_{t-1} - lr * g / (\sqrt{accum_g_t} + \epsilon)$$
  -
  - References
  - See [paper]
  - (http://www.jmlr.org/papers/volume12/duchi11a/duchi11a.pdf)
  - or this
  - [intro](https://ppasupat.github.io/a9online/uploads/proximal_notes.pdf).
  + $$accum_{g_t} := accum_{g_{t-1}} + g^2$$
  + $$\theta_t := \theta_{t-1} - lr * g / (\sqrt{accum_{g_t}} + \epsilon)$$
  +
  + References:
  +
  + * [Paper](http://www.jmlr.org/papers/volume12/duchi11a/duchi11a.pdf).
  + * [Introduction]
  + (https://ppasupat.github.io/a9online/uploads/proximal_notes.pdf).
  """
  def __init__(self,
  