Add support for encoder-only T5 models (#8900)

* gguf-py : add T5ENCODER model architecture

* common : call llama_decode() during warmup only if the model has decoder

* convert-hf : add T5EncoderModel

* llama : add llama_model_has_decoder() API function

* llama : split build_t5() into build_t5_encoder() and build_t5_decoder()

* llama : add support for LLM_ARCH_T5ENCODER

* llama-embedding : add support for LLAMA_POOLING_TYPE_NONE

* llama-embedding : add support for encoder-only models

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Co-authored-by: Stanisław Szymczyk <sszymczy@gmail.com>