message: Add bfloat16 support to Sigmoid CPU Op
  The inference converter has an option to convert both the TPU
  and CPU graphs to use bfloat16. It is converting a Sigmoid
  op on the CPU to bfloat16. This cl updates the Sigmoid op
  to support bfloat16.
diff: @@ -17,8 +17,8 @@ limitations under the License.
  #include "tensorflow/core/kernels/cwise_ops_gradients.h"
  namespace tensorflow {
  -REGISTER5(UnaryOp, CPU, "Sigmoid", functor::sigmoid, float, Eigen::half, double,
  - complex64, complex128);
  +REGISTER6(UnaryOp, CPU, "Sigmoid", functor::sigmoid, bfloat16, float,
  + Eigen::half, double, complex64, complex128);
  #if GOOGLE_CUDA || TENSORFLOW_USE_ROCM
  REGISTER3(UnaryOp, GPU, "Sigmoid", functor::sigmoid, float, Eigen::half,
  double);
  @@ -27,8 +27,8 @@ REGISTER3(UnaryOp, GPU, "Sigmoid", functor::sigmoid, float, Eigen::half,
  REGISTER(UnaryOp, SYCL, "Sigmoid", functor::sigmoid, float);
  #endif // TENSORFLOW_USE_SYCL
  -REGISTER5(SimpleBinaryOp, CPU, "SigmoidGrad", functor::sigmoid_grad, float,
  - Eigen::half, double, complex64, complex128);
  +REGISTER6(SimpleBinaryOp, CPU, "SigmoidGrad", functor::sigmoid_grad, bfloat16,
  + float, Eigen::half, double, complex64, complex128);
  #if GOOGLE_CUDA || TENSORFLOW_USE_ROCM
  REGISTER3(SimpleBinaryOp, GPU, "SigmoidGrad", functor::sigmoid_grad, float,
  Eigen::half, double);
  