Tpu embedding improvements: 1. Create the recv_tpu_embedding_activations ops in    TpuEmbeddingCollection.AddActivations() and cache the result, so we can allow    multiple lookup calls from the client. 2. Let EmbLookup[Sparse] only return the embeddings that are required by the input. 3. Fix some docstrings.
Tpu embedding improvements:
1. Create the recv_tpu_embedding_activations ops in
   TpuEmbeddingCollection.AddActivations() and cache the result, so we can allow
   multiple lookup calls from the client.
2. Let EmbLookup[Sparse] only return the embeddings that are required by the input.
3. Fix some docstrings.

PiperOrigin-RevId: 381401821

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