Missing validation causes denial of service via `UnsortedSegmentJoin`
### Impact
The implementation of `tf.raw_ops.UnsortedSegmentJoin`
https://github.com/tensorflow/tensorflow/blob/f3b9bf4c3c0597563b289c0512e98d4ce81f886e/tensorflow/core/kernels/unsorted_segment_join_op.cc#L92-L95 does not fully validate the input arguments. This results in a `CHECK`-failure which can be used to trigger a denial of service attack:
```python
import tensorflow as tf
tf.raw_ops.UnsortedSegmentJoin(
inputs=tf.constant("this", shape=[12], dtype=tf.string),
segment_ids=tf.constant(0, shape=[12], dtype=tf.int64),
num_segments=tf.constant(0, shape=[12], dtype=tf.int64))
```
The code assumes `num_segments` is a scalar but there is no validation for this before accessing its value:
```cc
const Tensor& num_segments_tensor = context->input(2);
OP_REQUIRES(context, num_segments_tensor.NumElements() != 0,
errors::InvalidArgument("Number of segments cannot be empty."));
auto num_segments = num_segments_tensor.scalar<NUM_SEGMENTS_TYPE>()();
```
### Patches
We have patched the issue in GitHub commit 13d38a07ce9143e044aa737cfd7bb759d0e9b400
https://github.com/tensorflow/tensorflow/commit/13d38a07ce9143e044aa737cfd7bb759d0e9b400.
The fix will be included in TensorFlow 2.9.0. We will also cherrypick this commit on TensorFlow 2.8.1, TensorFlow 2.7.2, and TensorFlow 2.6.4, 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.
### Attribution
This vulnerability has been reported by Neophytos Christou from Secure Systems Lab at Brown University.