convert: qwen2 from safetensors (#8408)

Add native support for converting Qwen2 family models (including Qwen2.5)
from safetensors to gguf format so we can run it.

convert/convert.go

@@ -187,6 +187,8 @@ func ConvertModel(fsys fs.FS, ws io.WriteSeeker) error {
 		conv = &gemma2Model{}
 	case "Phi3ForCausalLM":
 		conv = &phi3Model{}
+	case "Qwen2ForCausalLM":
+		conv = &qwen2Model{}
 	case "BertModel":
 		conv = &bertModel{}
 	default: