* Improve the perf of tf.sparse.segment_(mean/sum/sqrt) ops when using
* Improve the perf of tf.sparse.segment_(mean/sum/sqrt) ops when using
  bfloat16/float16 precision with float32 for aggregation.
* Use float32 accumulation for the gradients of
  tf.sparse.segment_(mean/sum/sqrt) ops when using bfloat16/float16 precision.
