Add straight-through pass in vq-vae, other small Transformer VAE improvements.
Add straight-through pass in vq-vae, other small Transformer VAE improvements.

PiperOrigin-RevId: 179222668

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