Refactor: convert_hf_to_gguf.py (#17114)

* move conversion code to a dedicated conversion directory and split the files akin to the src/models architecture

---------

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

conversion/maincoder.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+from .base import ModelBase, TextModel, gguf
+
+
+@ModelBase.register("MaincoderForCausalLM")
+class MaincoderModel(TextModel):
+    model_arch = gguf.MODEL_ARCH.MAINCODER
+
+    def set_gguf_parameters(self):
+        super().set_gguf_parameters()
+
+        if (head_dim := self.hparams.get("head_dim")) is not None:
+            self.gguf_writer.add_rope_dimension_count(head_dim)