refactor(项目结构) 重构为 workspace 多成员项目

- 将项目拆分为三个 crate：minicpm-core（核心模型）、minicpm-convert（转换功能）、minicpm-inference（推理功能）
- 添加两个示例：minimal-inference（最小推理）和 convert（模型转换）
- 转换后自动拷贝 config.json 和 tokenizer.json 到 model 目录
- 更新 README 说明 workspace 结构和使用方式

Cargo.toml

@@ -1,24 +1,3 @@
-[package]
-name = "minicpm-burn"
-version = "0.1.0"
-edition = "2021"
-
-[lib]
-name = "minicpm_burn_lib"
-path = "src/lib.rs"
-
-[[bin]]
-name = "minicpm-burn"
-path = "src/main.rs"
-
-[dependencies]
-burn = { version = "0.21", features = ["std", "wgpu"] }
-tokenizers = "0.20"
-serde = { version = "1.0", features = ["derive"] }
-serde_json = "1.0"
-anyhow = "1.0"
-rand = "0.8"
-memmap2 = "0.9"
-clap = { version = "4.5", features = ["derive"] }
-
-[dev-dependencies]
+[workspace]
+members = ["crates/minicpm-core", "crates/minicpm-convert", "crates/minicpm-inference", "examples/minimal-inference", "examples/convert"]
+resolver = "2"