Improved keras imports

[Rocketknight1]

A sneaky bug was hiding in our Keras imports, where an import for call_context appeared to succeed on some TF versions, but actually got an older, unusable version of the function. This caused build() to behave improperly in some cases.

I went on a quest to fix this, and generally clean up our version-specific imports for TensorFlow to stop things like this from happening in future. I also bumped the minimum version for TF to 2.6 (2.6 should be 2 years old by the time of our next release), and eliminated the version cap in our dependency table because TF 2.13 should also be fully supported now.

This involved a partial rewrite of some code, where we checked for KerasTensor in a lot of places. However, this is an internal class for Keras and they keep moving it around, so trying to import it feels like a bad idea. Instead, I'm relying on tf.is_tensor(), which returns True for anything tensor-y, including symbolic tensors and Keras Input placeholders.

Fixes #24437

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[ydshieh]

Keras and they keep moving it around

It's tensor-flow ... that's probably the reason.

[Rocketknight1]

I tested this with a spread of TF versions going back to 2.6. Even at 2.6, it was pretty hard to get an environment that worked with modern transformers - old TensorFlow keeps trying to use NumPy features that have been deprecated and deleted, but our modern libraries need a minimum version of NumPy to run at all, so there's actually only a very narrow window of NumPy versions that can even run both at once! I think going back to 2.5 or earlier would be very difficult, so I'm pretty comfortable with bumping our minimum version at this point.

In all versions I tested with this patch, our test suite runs well and the issue identified in #24437 is fixed, so this should be ready to go after it's reviewed!

[sgugger]

Thanks for the cleanup. I will agree to not put a maximum pin on TensorFlow when they stop making breaking changes in minor releases (so never 😅 )

[sgugger]

No let's leave a maximum pin. I do not want any surprises.

[Rocketknight1]

Deal, but I'm bumping the cap to <2.14, since 2.13 works fine!

[sgugger]

Sure!