Fix SymbolModality for prepend_inputs_masked_attention
Fix SymbolModality for prepend_inputs_masked_attention

PiperOrigin-RevId: 192638880

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