Add support for falcon-mamba (#1074)

* Add support for falcon-mamba

* nits

* nit

---------

Co-authored-by: Awni Hannun <awni@apple.com>

llms/README.md

@@ -221,6 +221,7 @@ Here are a few examples of Hugging Face models that work with this example:
 - [pfnet/plamo-13b-instruct](https://huggingface.co/pfnet/plamo-13b-instruct)
 - [stabilityai/stablelm-2-zephyr-1_6b](https://huggingface.co/stabilityai/stablelm-2-zephyr-1_6b)
 - [internlm/internlm2-7b](https://huggingface.co/internlm/internlm2-7b)
+- [tiiuae/falcon-mamba-7b-instruct](https://huggingface.co/tiiuae/falcon-mamba-7b-instruct)
 
 Most
 [Mistral](https://huggingface.co/models?library=transformers,safetensors&other=mistral&sort=trending),

llms/mlx_lm/models/mamba.py

@@ -23,6 +23,8 @@ class ModelArgs(BaseModelArgs):
     use_conv_bias: bool
     time_step_rank: int
     tie_word_embeddings: bool = True
+    use_bcdt_rms: bool = False
+    mixer_rms_eps: float = 1e-6
 
     def __post_init__(self):
         if not hasattr(self, "hidden_size") and hasattr(self, "d_model"):
@@ -44,6 +46,8 @@ class ModelArgs(BaseModelArgs):
 
         if self.time_step_rank == "auto":
             self.time_step_rank = math.ceil(self.hidden_size / 16)
+        if self.model_type == "falcon_mamba":
+            self.use_bcdt_rms = True
 
 
 class DepthWiseConv1d(nn.Module):
@@ -83,6 +87,11 @@ class MambaBlock(nn.Module):
         self.intermediate_size = args.intermediate_size
         self.time_step_rank = int(args.time_step_rank)
         self.use_conv_bias = args.use_conv_bias
+        self.use_bcdt_rms = args.use_bcdt_rms
+        if self.use_bcdt_rms:
+            self.mixer_norm = lambda x: mx.fast.rms_norm(
+                x, mx.ones(x.shape[-1], x.dtype), eps=args.mixer_rms_eps
+            )
 
         self.in_proj = nn.Linear(
             self.hidden_size, self.intermediate_size * 2, bias=args.use_bias
@@ -126,6 +135,8 @@ class MambaBlock(nn.Module):
             ],
             axis=-1,
         )
+        if self.use_bcdt_rms:
+            delta, B, C = map(self.mixer_norm, (delta, B, C))
         delta = nn.softplus(self.dt_proj(delta))
         new_state = mx.expand_dims(delta * x, -1) * mx.expand_dims(B, 1)
         if state is not None:

llms/mlx_lm/utils.py

@@ -29,6 +29,7 @@ from .tuner.utils import load_adapters
 MODEL_REMAPPING = {
     "mistral": "llama",  # mistral is compatible with llama
     "phi-msft": "phixtral",
+    "falcon_mamba": "mamba",
 }
 
 MAX_FILE_SIZE_GB = 5