Fix gradient path for tf.sparse.segment_{sum/mean/sqrt) and bfloat16/float16
Fix gradient path for tf.sparse.segment_{sum/mean/sqrt) and bfloat16/float16
operands. Use float32 accumulation for the gradients of the operation ops when
using bfloat16/float16 operands. This matches the behavior on the forward
pass, improving model accuracy.
