llama : support T5 models with unequal number of encoder-decoder layers (#15909)

llama : support T5 models with unequal number of encoder-decoder layers (#15909)

* Extend the support of T5 models with different encoder-decoder layers

Signed-off-by: Jie Fu <redacted>
* Update convert_hf_to_gguf.py

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update gguf-py/gguf/constants.py

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update gguf-py/gguf/gguf_writer.py

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-arch.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-arch.h

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-hparams.h

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/llama-model.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Rename n_dec_layer --> dec_n_layer

Signed-off-by: Jie Fu <redacted>
* Adapt to cases when dec_n_layer > n_layer

Signed-off-by: Jie Fu <redacted>
---------

Signed-off-by: Jie Fu <redacted>
Co-authored-by: Sigbjørn Skjæret <redacted>