def causal_attention_mask(nd, ns, dtype):
        """1's in the lower triangle, counting from the lower right corner.
        Same as tf.matrix_band_part(tf.ones([nd, ns]), -1, ns-nd), but doesn't produce garbage on TPUs.
        """
        i = tf.range(nd)[:, None]
        j = tf.range(ns)
        m = i >= j - ns + nd
        return tf.cast(m, dtype)
1's in the lower triangle, counting from the lower right corner.
        Same as tf.matrix_band_part(tf.ones([nd, ns]), -1, ns-nd), but doesn't produce garbage on TPUs.
        
xtune/src/transformers/modeling_tf_gpt2.py