Add DB model for text detection

examples/db/README.md

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+## Quick Start
+
+```shell
+cargo run -r --example db
+```
+
+## Or you can manully
+
+### 1. Donwload ONNX Model
+
+[ppocr-v3-db-dyn](https://github.com/jamjamjon/assets/releases/download/v0.0.1/ppocr-v3-db-dyn.onnx)
+[ppocr-v4-db-dyn](https://github.com/jamjamjon/assets/releases/download/v0.0.1/ppocr-v4-db-dyn.onnx)
+
+### 2. Specify the ONNX model path in `main.rs`
+
+```Rust
+let options = Options::default()
+    .with_model("ONNX_PATH")    // <= modify this
+    .with_profile(false);
+```
+
+### 3. Run
+
+```bash
+cargo run -r --example db
+```
+
+### Speed test
+
+| Model           | Image size | TensorRT<br />f16 | TensorRT<br />f32 | CUDA<br />f32 |
+| --------------- | ---------- | ----------------- | ----------------- | ------------- |
+| ppocr-v3-db-dyn | 640x640    | 1.8585ms          | 2.5739ms          | 4.3314ms      |
+| ppocr-v4-db-dyn | 640x640    | 2.0507ms          | 2.8264ms          | 6.6064ms      |
+
+***Test on RTX3060***
+
+## Results
+
+![](./demo.jpg)

examples/db/main.rs

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+use usls::{models::DB, DataLoader, Options};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // build model
+    let options = Options::default()
+        .with_model("../models/ppocr-v4-db-dyn.onnx")
+        .with_i00((1, 1, 4).into())
+        .with_i02((608, 640, 960).into())
+        .with_i03((608, 640, 960).into())
+        .with_confs(&[0.7])
+        .with_saveout("DB-Text-Detection")
+        .with_dry_run(5)
+        // .with_trt(0)
+        // .with_fp16(true)
+        .with_profile(true);
+    let mut model = DB::new(&options)?;
+
+    // load image
+    let x = DataLoader::try_read("./assets/math.jpg")?;
+
+    // run
+    let _y = model.run(&[x])?;
+
+    Ok(())
+}