Destroy Keras' multiprocessing pools to prevent deadlocks. (deprecated)

In general multiprocessing.Pool can interact quite badly with other, seemingly unrelated, parts of a codebase due to Pool's reliance on fork. This method cleans up all pools which are known to belong to Keras (and thus can be safely terminated).

grace_period Time (in seconds) to wait for process cleanup to propagate.
use_sigkill Boolean of whether or not to perform a cleanup