Gracefully disallow updating resource variables with invalid shapes.

During graph construction, the shape function for AssignAddVariableOp etc.
would raise an error when the value being "assign add"ed to the variable
has an incompatible shape.

With eager execution, no such validation was being made which triggerred
an assertion failure in eigen:
https://github.com/eigenteam/eigen-git-mirror/blob/7d97e1cbbe4424fda39e31c88def7c0863897640/unsupported/Eigen/CXX11/src/Tensor/TensorEvaluator.h#L479

This change prevents that assertion failure.

PiperOrigin-RevId: 215749071