Add initial T5Gemma S/S support

[maasir554]

Summary

Adds initial T5Gemma S/S support to gemma.cpp.

This includes:

• T5Gemma S/S model config and tensor metadata
• HF safetensors -> SBS conversion through python/convert_from_safetensors.py
• Encoder-decoder runtime path for fresh seq2seq generation
• BF16 default conversion path for correctness/parity
• Basic docs and focused config/tensor/cache tests

Scope

This PR targets T5Gemma v1 S/S only.

Out of scope:

• T5Gemma 2
• Other T5Gemma sizes
• Multiturn/cache reuse with changing encoder inputs
• Fully optimized T5-specific attention/logits kernels

Local Validation

• bazel test -c opt //:configs_test //:tensor_info_test //:kv_cache_test
• Local optimized smoke test with t5gemma-s-s-it
  • prompt: Hello
  • output begins: I am a new user, ...
  • local speed: roughly 30-35 generated tok/s on MacBook M1, 8GB RAM

Notes

Opened as draft to get early maintainer feedback on scope, naming, and whether this should be split further.

Additional Note: //:gemma_lib_test currently fails locally for me in an existing GemmaAttentionFlash/EMU128 golden mismatch on Apple Silicon; this reproduced outside the T5Gemma changes.

Related to #661

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[jan-wassenberg]

Remarkably nice work, very clean code, congrats :) A few suggestions:

[maasir554]

Remarkably nice work, very clean code, congrats :) A few suggestions:

Thank you, really appreciate the review and kind words :)
I’ll address your suggestions in a follow-up update.

[maasir554]

Hi, I addressed the suggestions in the latest push:

• added a shared Activations::num_layers
• made encoder/decoder config fields conditional on is_encoder_decoder
• moved/reused T5 token IDs via tokenizer.h / BOS_ID
• reused MaybeLogitsSoftCap
• hoisted T5Gemma encoder scratch buffers into Activations
• switched internal encoder-cache checks to HWY_DASSERT

Thanks again for the review!

[jan-wassenberg]

Thanks for updating :) A few more questions and one fix:

[maasir554]

Thanks a lot for the detailed review and suggestions :)

I’ve pushed a follow-up commit addressing the comments. A couple of these were cleanup mistakes on my side, especially the temporary single-threading change and the unrelated Bazel flag removal. I’m still new to this codebase, so the review was really helpful for understanding the project conventions better.

Thanks again for taking the time to explain the preferred patterns.