[tensorflow] Add support for the `is_symbolic_tensor` predicate
Author: hvaara
Pull request: yes

# What does this PR do?

This PR adds support for the `is_symbolic_tensor` predicate in TensorFlow. This predicate will become available starting with version 2.14.

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

cc @Rocketknight1 

This looks clean! Do you have any links to documentation/discussion about `is_symbolic_tensor` in TF, though? This is the first I've heard of it - I can see the relevant code in the TF codebase, but are we guaranteed that the behaviour won't change before the 2.14 release? Also, given that it's in the codebase already, won't it be in the 2.13 release rather than 2.14?

I wrote the `is_symbolic_tensor` code in question. The 2.14 was a guess from me, I wasn't sure if the original PR was going to make the TF branch cut, but it looks like it will make 2.13. The intention of `is_symbolic_tensor` is actually to provide more stability:

We're looking into breaking the current inheritance setup in TF. EagerTensor inherits from symbolic Tensor, and this adds a lot of weird complication in the TF codebase, along with awkward checks like type(t) == Tensor. This method was introduced to avoid churning the few users who need to distinguish between eager and symbolic tensors.

We don't have a proposal yet for the split, but we're front-running some of this prep/cleanup.

LGTM in that case, and thanks for the clarification!