TF: tf.debugging assertions without tf.running_eagerly() protection
# What does this PR do?

In most cases, `tf.debugging` assertions (which exist as normal asserts in PyTorch models) are protected by a check for eager execution. In some places, we have stated in comments that these ops do not work with XLA, hence the check for eager execution.

The actual state of `tf.debugging` is the following:
- They never cause crashes, not even in XLA;
- In XLA, they are not executed.

Since they are innocuous, this PR removes the checks for eager execution. Non-XLA graph-mode model calls will now benefit from these checks (e.g. when a user calls `model.fit()`)
_The documentation is not available anymore as the PR was closed or merged._