`CHECK`-fails due to attempting to build a reference tensor

Impact

A malicious user can cause a denial of service by altering a SavedModel such that [Grappler optimizer would attempt to build a tensor using a reference dtype](https://github.com/tensorflow/tensorflow/blob/a1320ec1eac186da1d03f033109191f715b2b130/tensorflow/core/grappler/optimizers/constant_folding.cc#L1328-L1402). This would result in a crash due to a CHECK-fail [in the Tensor constructor](https://github.com/tensorflow/tensorflow/blob/a1320ec1eac186da1d03f033109191f715b2b130/tensorflow/core/framework/tensor.cc#L733-L781) as reference types are not allowed.

Patches

We have patched the issue in GitHub commit [6b5adc0877de832b2a7c189532dbbbc64622eeb6](https://github.com/tensorflow/tensorflow/commit/6b5adc0877de832b2a7c189532dbbbc64622eeb6).

The fix will be included in TensorFlow 2.8.0. We will also cherrypick this commit on TensorFlow 2.7.1, TensorFlow 2.6.3, and TensorFlow 2.5.3, as these are also affected and still in supported range.

For more information

Please consult [our security guide](https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md) for more information regarding the security model and how to contact us with issues and questions.