Add support for cohere2 (#1157)

* add support for cohere2

* revert to act_fn to silu

* fix tests and sliding window attention

* add tests

* add to tuner

* fix sliding window

* add coauthor :)

Co-authored-by: n8programs <43304488+N8python@users.noreply.github.com>

* Add rotating kvcache to save space

* some nits

* style

* nits

---------

Co-authored-by: n8programs <43304488+N8python@users.noreply.github.com>
Co-authored-by: N8 <n8@n8programs.com>
Co-authored-by: Awni Hannun <awni@apple.com>

llms/mlx_lm/tuner/utils.py

@@ -96,6 +96,7 @@ def linear_to_lora_layers(
         "gemma2",
         "starcoder2",
         "cohere",
+        "cohere2",
         "minicpm",
         "deepseek",
         "olmo2",