Allow problems with a fixed length to override max_length.  Restore previous functionality of using max_length=batch_size when hparams.max_length==0.
Allow problems with a fixed length to override max_length.  Restore previous functionality of using max_length=batch_size when hparams.max_length==0.

PiperOrigin-RevId: 180980913

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