C++ gradients: Fractional*Pool, Soft{Plus,Sign}

1. Adds gradients for four nn ops:
   FractionalAvgPool
   FractionalMaxPool
   SoftPlus
   SoftSign

2. Update randomization to allow numeric gradient
   checks on max pooling algorithms with more than
   one pool.

Resolves https://github.com/tensorflow/tensorflow/issues/17330