do not pull in tensorboard's keras callback

[haohuanw]

i am moving some of my models to keras3 and using jax backend for training. i am on a slightly older tensorboard (2.12) so when it pulls in keras callback through from tensorboard.plugins.hparams import api it tries to pull in keras2 since api also import keras callback in tensorboard repo.

this pr basically adjusted import to avoid accidentally pulling in keras in clu.

[haohuanw]

@andsteing would you be able to help me with reviewing the change? thanks!