Add LoRA linear definition (#12861)

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@diff-train-skip-merge

Co-authored-by: lucylq <lfq@meta.com>

Author: pytorchbot

examples/models/llama/TARGETS

@@ -13,6 +13,7 @@ runtime.python_library(
     name = "llama_transformer",
     srcs = [
         "llama_transformer.py",
+        "lora.py",
         "rope.py",
         "attention.py",
         "model_args.py",

examples/models/llama/attention.py

@@ -5,6 +5,7 @@
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
+from executorch.examples.models.llama.lora import LoRALinear
 from executorch.examples.models.llama.model_args import ModelArgs
 from executorch.examples.models.llama.norm import RMSNorm
 from executorch.examples.models.llama.rope import Rope
@@ -325,7 +326,20 @@ def update(
 
 @register_attention("mha")
 class AttentionMHA(Attention):
-    def __init__(self, args: ModelArgs, layer_id: int, rope: Rope):
+    def __init__(
+        self,
+        args: ModelArgs,
+        layer_id: int,
+        rope: Rope,
+    ):
+        """
+        Multi-head attention layer.
+
+        Args:
+            args (ModelArgs): Model configuration parameters.
+            layer_id (int): Layer index.
+            rope (Rope): Rotary position embedding module.
+        """
         super().__init__()
         self.use_kv_cache = args.use_kv_cache
         self.n_heads = args.n_heads
@@ -350,16 +364,60 @@ def __init__(self, args: ModelArgs, layer_id: int, rope: Rope):
             self.q_norm_fn = RMSNorm(q_norm_dim, eps=args.norm_eps)
             self.k_norm_fn = RMSNorm(k_norm_dim, eps=args.norm_eps)
 
-        self.wq = nn.Linear(
-            self.dim, self.n_heads * self.head_dim, bias=self.attention_qkv_bias
+        self.wq = (
+            LoRALinear(
+                in_dim=args.dim,
+                out_dim=args.n_heads * args.head_dim,
+                rank=args.r,
+                alpha=args.lora_alpha,
+                dropout=0.0,
+                use_bias=args.attention_qkv_bias,
+            )
+            if args.target_modules is not None and "q_proj" in args.target_modules
+            else nn.Linear(
+                self.dim, self.n_heads * self.head_dim, bias=self.attention_qkv_bias
+            )
         )
-        self.wk = nn.Linear(
-            self.dim, self.n_kv_heads * self.head_dim, bias=self.attention_qkv_bias
+        self.wk = (
+            LoRALinear(
+                in_dim=args.dim,
+                out_dim=args.n_kv_heads * args.head_dim,
+                rank=args.r,
+                alpha=args.lora_alpha,
+                dropout=0.0,
+                use_bias=args.attention_qkv_bias,
+            )
+            if args.target_modules is not None and "k_proj" in args.target_modules
+            else nn.Linear(
+                self.dim, self.n_kv_heads * self.head_dim, bias=self.attention_qkv_bias
+            )
         )
-        self.wv = nn.Linear(
-            self.dim, self.n_kv_heads * self.head_dim, bias=self.attention_qkv_bias
+        self.wv = (
+            LoRALinear(
+                in_dim=args.dim,
+                out_dim=args.n_kv_heads * args.head_dim,
+                rank=args.r,
+                alpha=args.lora_alpha,
+                dropout=0.0,
+                use_bias=args.attention_qkv_bias,
+            )
+            if args.target_modules is not None and "v_proj" in args.target_modules
+            else nn.Linear(
+                self.dim, self.n_kv_heads * self.head_dim, bias=self.attention_qkv_bias
+            )
+        )
+        self.wo = (
+            LoRALinear(
+                in_dim=args.n_kv_heads * args.head_dim,
+                out_dim=args.dim,
+                rank=args.r,
+                alpha=args.lora_alpha,
+                dropout=0.0,
+                use_bias=args.attention_qkv_bias,
+            )
+            if args.target_modules is not None and "output_proj" in args.target_modules
+            else nn.Linear(self.n_heads * self.head_dim, self.dim, bias=False)
         )
-        self.wo = nn.Linear(self.n_heads * self.head_dim, self.dim, bias=False)
 
         self.layer_id = layer_id
 

examples/models/llama/lora.py

@@ -0,0 +1,48 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import torch
+from torch import nn
+
+
+class LoRALinear(nn.Module):
+    """LoRA linear layer as introduced in `LoRA: Low-Rank Adaptation of Large Language Models <https://arxiv.org/abs/2106.09685>`."""
+
+    def __init__(
+        self,
+        in_dim: int,
+        out_dim: int,
+        rank: int,
+        alpha: float,
+        dropout: float = 0.0,
+        use_bias: bool = False,
+    ):
+        super().__init__()
+        self.in_dim = in_dim
+        self.out_dim = out_dim
+        self.rank = rank
+        self.alpha = alpha
+        self.use_bias = use_bias
+        self.dropout = dropout
+
+        linear = nn.Linear(in_dim, out_dim, bias=use_bias)
+        weight = linear.weight
+        bias = linear.bias if self.use_bias else None
+        self.register_parameter("weight", nn.Parameter(weight))
+        self.register_parameter(
+            "bias", nn.Parameter(bias) if bias is not None else None
+        )
+
+        self.dropout = nn.Dropout(p=dropout) if dropout > 0.0 else nn.Identity()
+        self.lora_a = nn.Linear(in_features=in_dim, out_features=rank, bias=False)
+        self.lora_b = nn.Linear(in_features=rank, out_features=out_dim, bias=False)
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        out = torch.nn.functional.linear(x, self.weight, self.bias)
+        lora_out = self.lora_a(self.dropout(x))
+        lora_out = (self.alpha / self.rank) * self.lora_b(lora_out)
+
+        return out + lora_out

examples/models/llama/model_args.py

@@ -55,8 +55,18 @@ class ModelArgs:
     eos_count: int = 2
 
     quantization_args: Optional[dict] = None
+    # LoRA for QAT.
     lora_args: Optional[dict] = None
 
+    # LoRA arguments to set up a LoRA inference model.
+    # These arguments come directly from a torchtune LoRA config.
+    r: Optional[int] = None  # Rank.
+    lora_alpha: Optional[int] = None  # Alpha.
+    # Eg. q_proj, k_proj, v_proj, output_proj
+    target_modules: Optional[list] = None
+    peft_type: Optional[str] = None  # PEFT type.
+    base_model_name_or_path: Optional[str] = None  # Base model name or path.
+
     def __post_init__(self):
         if self.n_kv_heads is None:
             self.n_kv_heads = self.n_heads