[RFC][Android] Consolidate LlmModule constructor and generate() overloads

🐛 Describe the bug

Problem

LlmModule has 8 constructors and 10 generate() overloads. A codebase-wide search (executorch + executorch-examples) shows most are unused.

Constructors — 4 of 8 have zero call sites

Constructor	Call sites
(String, String, float)	3 (benchmark, instrumentation test, sanity check)
(int, String, String, float)	1 (LlamaDemo, no dataPath)
(int, String, String, float, String)	1 (LlamaDemo, with dataPath)
(int, String, String, float, List<String>)	2 (LlamaDemo LoRA)
(int, String, String, float, List<String>, int, int)	0 (only internal delegate target)
(int, String, String, float, String, int, int)	0
(String, String, float, String)	0
(LlmModuleConfig)	0

generate() — 7 of 10 have zero call sites

Overload	Call sites
(String, LlmCallback)	2
(String, int, LlmCallback)	2
(String, int, LlmCallback, boolean)	4
(String, LlmCallback, boolean)	0
(String, int, LlmCallback, boolean, float, int, int)	0 (native JNI entry, only internal)
(String, LlmGenerationConfig, LlmCallback)	0
All 4 image (int[], int, int, int, String, ...) variants	0 (image input now goes through prefillImages() + text-only generate())

LlmModuleConfig and LlmGenerationConfig builder classes exist but neither is used anywhere — all callers use the direct constructors/overloads instead.

Proposal

Phase 1: Deprecate and redirect

1. Keep one constructor: LlmModule(LlmModuleConfig). Deprecate all others with @Deprecated pointing to the config builder.

2. Keep two generate() signatures:

   • generate(String prompt, LlmGenerationConfig config, LlmCallback callback) — the config-based entry point
   • generate(String prompt, int seqLen, LlmCallback callback, boolean echo, float temperature, int numBos, int numEos) — the native JNI method (package-private or private)

   Deprecate all other generate() overloads.

3. Remove all image-based generate() overloads — they are dead code. Multimodal input already uses the prefillImages()/prefillAudio()/prefillPrompt() → generate() flow.

4. Fix LlmModuleConfig.getDataPath() to return List<String> instead of String, matching what the constructor actually needs.

5. Deduplicate model type constants — remove MODEL_TYPE_* from LlmModule, keep only in LlmModuleConfig.

Phase 2: Remove (next release)

Remove all deprecated overloads.

Migration

All existing call sites migrate to:

// Before
LlmModule module = new LlmModule(MODEL_TYPE_TEXT, modelPath, tokenizerPath, 0.8f);
module.generate(prompt, seqLen, callback, false);

// After
LlmModule module = new LlmModule(
    LlmModuleConfig.create()
        .modulePath(modelPath)
        .tokenizerPath(tokenizerPath)
        .temperature(0.8f)
        .build());
module.generate(prompt, LlmGenerationConfig.create()
    .seqLen(seqLen)
    .echo(false)
    .build(), callback);

Compatibility

LlmModule is annotated @Experimental ("subject to change without notice"), so breaking changes are acceptable. A deprecation phase is still recommended since external apps may use these APIs.

Scope

Java-only change. No JNI or C++ changes needed — the single native generate() method stays as-is.

Versions

0d9799f

0d9799f

cc @cbilgin