Fix dtype mismatch in RoPE cos/sin for mixed precision training

Replace F.silu with explicit SiLU implementation in SwiGLUFeedForward

Replace F.scaled_dot_product_attention with explicit implementation

Remove dead attn_dropout layer from GroupedQueryAttention

refactor(model): replace single-letter vars with descriptive names for readability

docs: translate all Korean comments and docstrings to English

Initial commit: LLM-1B-Lab project setup