Introduce MultimethodLoraConfig (pytorch#17455)

This PR was created by the merge bot to help merge the original PR into
the main branch.
ghstack PR number: pytorch#17230 by
@lucylq
^ Please use this as the source of truth for the PR details, comments,
and reviews
ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/lucylq/133/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/lucylq/133/head
Merge bot PR base:
https://github.com/pytorch/executorch/tree/gh/lucylq/132/orig
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/lucylq/133/orig
Differential Revision:
[D92315627](https://our.internmc.facebook.com/intern/diff/D92315627/)
@diff-train-skip-merge

---------

Co-authored-by: Github Executorch <github_executorch@arm.com>
Co-authored-by: lucylq <lfq@meta.com>

Author: 3 people

extension/llm/export/config/llm_config.py

@@ -22,7 +22,7 @@
 import re
 from dataclasses import dataclass, field
 from enum import Enum
-from typing import ClassVar, List, Optional
+from typing import ClassVar, Dict, List, Optional
 
 
 ################################################################################
@@ -287,6 +287,37 @@ class DebugConfig:
     verbose: bool = False
 
 
+################################################################################
+############################## MultimethodLoraConfig ###########################
+################################################################################
+
+
+@dataclass
+class MultimethodLoraConfig:
+    """Configuration for exporting multiple methods to a single .pte file.
+
+    Maps method names to optional LoRA configurations. A None value means
+    the method uses base model weights.
+
+    Attributes:
+        methods: Dict mapping method names to optional LoRA configs.
+            Empty dict disables multimethod export.
+
+    Example:
+        MultimethodLoraConfig(methods={
+            "forward": None,  # base model
+            "lora_forward": lora_config,  # LoRA variant
+        })
+    """
+
+    methods: Dict[str, Optional[LoraConfig]] = field(default_factory=dict)
+
+    @property
+    def enabled(self) -> bool:
+        """Returns True if multimethod_lora export is configured."""
+        return len(self.methods) > 0
+
+
 ################################################################################
 ############################# QuantizationConfig ###############################
 ################################################################################
@@ -543,6 +574,9 @@ class LlmConfig:
     model: ModelConfig = field(default_factory=ModelConfig)
     export: ExportConfig = field(default_factory=ExportConfig)
     debug: DebugConfig = field(default_factory=DebugConfig)
+    multimethod_lora: MultimethodLoraConfig = field(
+        default_factory=MultimethodLoraConfig
+    )
     quantization: QuantizationConfig = field(default_factory=QuantizationConfig)
     backend: BackendConfig = field(default_factory=BackendConfig)