Prepare 2.3.1 release

Update README

Fix sklearn regressor test?

Prepare 2.2.4 release.

[RELNOTES] [P] Write to TensorBoard every x samples. (keras-team#11152)

* Working on improving tensor flow callbacks

* Adding batch level TensorBoard logging (implementing the `on_batch_end` method to the TensorBoard class

* Interim commit -- added notes.

* Corrected stylistic issues -- brought to compliance w/ PEP8

* Added the missing argument in the test suite.

* Added the possibility to choose how frequently tensorboard should log
the metrics and losses.

* Fixed the issue of the validation data not being displayed.

* Fixed the issue about the callback not remembering when was the last
time it wrote to the logs.

* Removed the error check.

* Used update_freq instead of write_step.

* Forgot to change the constructor call.

Prepare new release.

Prepare 2.2.1 release.

Style fix

fixing typos (keras-team#10016)

Prepare 2.1.5 release