Adds a new (local) modality: SigmoidClassSymbolModality for performing binary (sigmoid_cross_entropy_with_logits) classification.
Adds a new (local) modality: SigmoidClassSymbolModality for performing binary (sigmoid_cross_entropy_with_logits) classification.

PiperOrigin-RevId: 189977484

vthorsteinsson/tensor2tensor,tensorflow/tensor2tensor,vthorsteinsson/tensor2tensor,vthorsteinsson/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,vthorsteinsson/tensor2tensor,tensorflow/tensor2tensor