add: embedding model (#40694)
* Gemma 3 for Embeddings
* Style fixes
* Rename conversion file for consistency
* Default padding side emb vs gen
* Corrected 270m config
* style fixes
* EmbeddingGemma config
* TODO for built-in prompts
* Resolving the sentence similarity bug and updating the architecture
* code style
* Add query prompt for SentenceTransformers
* Code quality
* Fixing or_mask_function return types
* Adding placeholder prompts for document and passage
* Finalizing prompt templates
* Adding Retrieval ro preconfigured prompts
* Add Gemma 3 270M Config
* Correcting num_linear_layers flag default
* Export Sentence Transformer in correct dtype
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Co-authored-by: Sindhu Raghuram <sindhuraghuram@google.com>
