Support for slerp merging models (#455)

* support for slerp merging models

* docs

* update docs

* format'

llms/mlx_lm/MERGE.md

@@ -0,0 +1,50 @@
+# Model Merging
+
+You can use `mlx-lm` to merge models and upload them to the Hugging
+Face hub or save them locally for LoRA fine tuning.
+
+The main command is `mlx_lm.merge`:
+
+```shell
+python -m mlx_lm.merge --config config.yaml 
+```
+
+The merged model will be saved by default in `mlx_merged_model`. To see a
+full list of options run:
+
+```shell
+python -m mlx_lm.merge --help
+```
+
+Here is an example `config.yaml`:
+
+```yaml
+models:
+  - OpenPipe/mistral-ft-optimized-1218
+  - mlabonne/NeuralHermes-2.5-Mistral-7B
+method: slerp
+parameters:
+  t:
+    - filter: self_attn
+      value: [0, 0.5, 0.3, 0.7, 1]
+    - filter: mlp
+      value: [1, 0.5, 0.7, 0.3, 0]
+    - value: 0.5
+```
+
+The `models` field is a list of Hugging Face repo ids. The first model in the
+list is treated as the base model into which the remaining models are merged.
+
+The `method` field is the merging method. Right now `slerp` is the only
+supported method.
+
+The `parameters` are the corresponding parameters for the given `method`.
+Each parameter is a list with `filter` determining which layer the parameter
+applies to and `value` determining the actual value used. The last item in
+the list without a `filter` field is the default.
+
+If `value` is a list, it specifies the start and end values for the
+corresponding segment of blocks. In the example above, the models have 32
+blocks. For blocks 1-8, the layers with `self_attn` in the name will use the
+values `np.linspace(0, 0.5, 8)`, the same layers in the next 8 blocks (9-16)
+will use `np.linspace(0.5, 0.3, 8)`, and so on.