[CausalLM] Extract TransformerBase abstract class and refactor run()

[EunjuYang]

Summary

Port of upstream nntrainer/nntrainer#3827 into Quick.AI as requested in #15.

Introduces an abstract TransformerBase class so that multiple Transformer
variants (e.g. NNTrainer-based Transformer, future QNNTransformer) can
share a common interface without inheriting HuggingFace-specific config
parsing. Also refactors run() to remove do_sample from the public
interface and add a void* output_buf parameter for retrieving results.

Resolves #15.
Upstream reference: nntrainer/nntrainer#3827

Changes

1. Extract TransformerBase (models/transformer_base.h)

   • Pure-virtual initialize(), load_weight(), save_weight(), run()
   • Move shared state (model, tokenizer, NUM_VOCAB, DIM,
     NUM_LAYERS, MAX_SEQ_LEN, BATCH_SIZE, INIT_SEQ_LEN,
     NUM_TO_GENERATE) and helpers (LoadJsonFile, ModelType,
     LayerHandle/ModelHandle aliases) here
   • Transformer now inherits from TransformerBase
   • Factory::Creator returns unique_ptr<TransformerBase>

2. Refactor run() signature

   • Drop do_sample from the interface — CausalLM now reads it from
     generation_config.json during setupParameters() and stores it as
     DO_SAMPLE
   • Add void* output_buf = nullptr for caller-side result retrieval:
     • CausalLM fills std::vector<std::string>*
     • SentenceTransformer fills std::vector<float*>*
     • nullptr keeps existing stdout-only behavior
   • Add simple overload run(prompt, output_buf) that delegates to the
     full version with empty system/tail prompts

Quick.AI-specific adaptations (vs upstream PR)

• Used quick_dot_ai namespace (instead of upstream's causallm)
• Preserved Quick.AI's existing save_weight(path, dtype, layer_dtype_map)
  overload on Transformer (used by quantize.cpp)
• Exposed that overload as a virtual on TransformerBase with a default
  throw std::runtime_error("not implemented") so the call site through
  Factory::create() resolves cleanly while non-NNTrainer subclasses
  (e.g. future QNN) are not forced to implement it

Files changed

• New: models/transformer_base.h
• Modified: api/causal_lm_api.cpp, factory.h, main.cpp,
  models/causal_lm.{h,cpp}, models/sentence_transformer.{h,cpp},
  models/transformer.{h,cpp}

[baek2sm]

LGTM

[dlwlzzero]

LGTM