message: Improve fast_tensor_util for bfloat16
  In 19180, improvement has been done to speed up the
  fast_tensor_util for `float16`. As both `float16`
  and `bfloat16` uses the same size, `bfloat16`
  could be improved as well. This fix speeds up `bfloat16`
  in a similiar fashion as `float16`.
  This fix is related to 19180.
diff: @@ -6,6 +6,13 @@ cimport numpy as np
  from tensorflow.python.util import compat
  +def AppendBFloat16ArrayToTensorProto(
  + tensor_proto, np.ndarray[np.uint16_t, ndim=1] nparray):
  + cdef long i, n
  + n = nparray.size
  + for i in range(n):
  + tensor_proto.half_val.append(nparray[i])
  +
  def AppendFloat16ArrayToTensorProto(
  # For numpy, npy_half is a typedef for npy_uint16,
  