CI: Unpin tf-nightly and stay on Keras 2

[yatbear]

tf-nightly was pinned at #6655 due to Keras 3 release.

Keras 2 <> Keras incompatibilities (keras-team/keras#18467) are impacting the Graph plugin and Hparams plugin, since the two plugins are not being actively developed and updating the code to be Keras 3 compatible is non-trivial, enforce using Keras 2 for the time being.

Note that tf-nightly is not compatible with tf-keras and needs to be used with tf-keras-nightly instead.

Googlers, see b/309503942 for more context.

Tested internally: cl/582334596

#oncall

[nfelt]

Thanks for getting this working again!

[nfelt]

Let's also add the dependency on tf-keras to the requirements.txt file, since there is one case where it's used beyond tests (in hparams/_keras.py).

[yatbear]

Good catch! Thanks!

[nfelt]

Actually now that it's in the requirements.txt, I think we might need some adjustments here to the CI to avoid having both tf-keras and tf-keras-nightly installed since I would assume they probably conflict.

Perhaps the simplest would just be to skip depending on tf-keras-nightly at all (i.e. no special handling for it here). We install nightly TF in our CI, but I think that's mostly a special case given our close coordination with TF in the past and the need to anticipate breakages before release deadlines (plus it's easier, since TF isn't in our pip requirements). We don't depend on pre-releases for any other pip dependencies, so we could probably skip it for tf-keras as well.

[nfelt]

Ah hmmm. But maybe we can't do the simple version, if tf-keras itself breaks with tf-nightly?

[yatbear]

Yea, I think we have to use tf-keras-nightly when using tf-nightly: b/306638603#comment15 :(

[nfelt]

Ugh, ok. Maybe we can do a hacky find/replace on requirements.txt before we pip install from it then?

[yatbear]

Done. Thanks for the suggestion! Hope it doesn't look too hacky 😞

[nfelt]

Haha it looks great - sometimes a hack is the best option 😉