convert : register UMT5Model architecture for T5 conversion (#17160)

Register UMT5Model as a supported architecture variant for T5 model conversion.
This allows the conversion to work for models downloaded with AutoModel.

convert_hf_to_gguf.py

@@ -7354,6 +7354,7 @@ class PLMModel(TextModel):
 @ModelBase.register("T5ForConditionalGeneration")
 @ModelBase.register("MT5ForConditionalGeneration")
 @ModelBase.register("UMT5ForConditionalGeneration")
+@ModelBase.register("UMT5Model")
 class T5Model(TextModel):
     model_arch = gguf.MODEL_ARCH.T5