**debug feature** for debugging numeric instability, like nan or infinity

[bela127]

System information

• TensorFlow version (you are using): nightly
• Are you willing to contribute it (Yes/No): cant do it alone, but willing to help

Describe the feature and the current behaviour/state.
I propose a debug feature for debugging numeric instability. The functionality should be similar to tf.debugging.enable_check_numerics() but more granular, but still more high level then tf.debugging.check_numerics.

Implementation Idea:
A tf.debugging.check_func_numerics annotation is added. It can be applied to ops warped in a python eager function and to tf.functons.
It will add numerics checks to all ops used in this function and, this is IMPORTANT, to all corresponding gradient ops.

The part of a model which courses numeric instability can be identified with this.

Will this change the current api? How?
it will add another debugging function/annotation, so the standard api is not changed.

Who will benefit with this feature?
Everyone with problems with nan and infinity in there models.
Especially for big models where tf.debugging.check_func_numerics can not be applied to the hole model.