docs: update HOWTO‑add‑model.md for ModelBase

This patch updates the example in docs/development/HOWTO-add-model.md to
use `@ModelBase.register("MyModelForCausalLM")` and
`class MyModel(ModelBase)` instead of the outdated `Model` base class.

Signed-off-by: Wook Song <[email protected]>

Author: wooksong

docs/development/HOWTO-add-model.md

@@ -23,11 +23,11 @@ The convert script reads the model configuration, tokenizer, tensor names+data a
 
 The required steps to implement for an HF model are:
 
-1. Define the model `Model.register` annotation in a new `Model` subclass, example:
+1. Define the model `ModelBase.register` annotation in a new `ModelBase` subclass, example:
 
 ```python
-@Model.register("MyModelForCausalLM")
-class MyModel(Model):
+@ModelBase.register("MyModelForCausalLM")
+class MyModel(ModelBase):
     model_arch = gguf.MODEL_ARCH.MYMODEL
 ```
 
@@ -77,7 +77,7 @@ block_mappings_cfg: dict[MODEL_TENSOR, tuple[str, ...]] = {
 Depending on the model configuration, tokenizer, code and tensors layout, you will have to override:
 - `Model#set_gguf_parameters`
 - `Model#set_vocab`
-- `Model#write_tensors`
+- `ModelBase#modify_tensors`
 
 NOTE: Tensor names must end with `.weight` or `.bias` suffixes, that is the convention and several tools like `quantize` expect this to proceed the weights.