Improper Input Validation

Summary

In Tensorflow before versions 1.15.4, 2.0.x before 2.0.3, 2.1.x before 2.1.2, 2.2.x before 2.2.1 and 2.3.x before 2.3.1, the `SparseFillEmptyRowsGrad` implementation has incomplete validation of the shapes of its arguments. Although `reverse_index_map_t` and `grad_values_t` are accessed in a similar pattern, only `reverse_index_map_t` is validated to be of proper shape. Hence, malicious users can pass a bad `grad_values_t` to trigger an assertion failure in `vec`, causing denial of service in serving installations.