Generalize optimizers in TPUEmbeddingLayer  - Remove hardcoded Adagrad  - Represent tpu_embedding.py library optimizers with layers  - Prepare for adding SGD, further optimizers
Generalize optimizers in TPUEmbeddingLayer
 - Remove hardcoded Adagrad
 - Represent tpu_embedding.py library optimizers with layers
 - Prepare for adding SGD, further optimizers

PiperOrigin-RevId: 328205226

tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo