Updated candle-book: Introduction, Installation, MNIST guide, and added CONTRIBUTING.md (#2897)

* added CONTRIBUTING.md to candle-book

* added description to candle-book introduction

* Updated formatting and added different features to candle-book installation

* mnist guide first draft candle-book

* updated mnist guide syntax and grammar for candle-book

* changed HelloWorld - Mnist to Tutorial - Mnist in SUMMARY.md

* updated intro to mnist guide in candle-book

candle-book/src/guide/mnist/training.md

@@ -0,0 +1,134 @@
+# Candle MNIST Tutorial
+
+## Training Implementation
+
+First, let's create a utility function `make_linear` that accepts a `VarBuilder` and returns an initialized linear layer. The `VarBuilder` constructs a `VarMap`, which is the data structure that stores our trainable parameters.
+
+```rust
+use candle_core::{Device, Result, Tensor};
+use candle_nn::{Linear, Module, VarBuilder, VarMap};
+
+fn make_linear(vs: VarBuilder, in_dim: usize, out_dim: usize) -> Result<Linear> {
+    let ws = vs.get_with_hints(
+        (out_dim, in_dim),
+        "weight",
+        candle_nn::init::DEFAULT_KAIMING_NORMAL,
+    )?;
+    let bound = 1. / (in_dim as f64).sqrt();
+    let bs = vs.get_with_hints(
+        out_dim,
+        "bias",
+        candle_nn::Init::Uniform {
+            lo: -bound,
+            up: bound,
+        },
+    )?;
+    Ok(Linear::new(ws, Some(bs)))
+}
+```
+
+Next, let's implement a `new` method for our model class to accept a `VarBuilder` and initialize the model. We use `VarBuilder::pp` to "push prefix" so that the parameter names are organized hierarchically: the first layer weights as `first.weight` and `first.bias`, and the second layer weights as `second.weight` and `second.bias`.
+
+```rust
+impl Model {
+    fn new(vs: VarBuilder) -> Result<Self> {
+        const IMAGE_DIM: usize = 784;
+        const HIDDEN_DIM: usize = 100;
+        const LABELS: usize = 10;
+
+        let first = make_linear(vs.pp("first"), IMAGE_DIM, HIDDEN_DIM)?;
+        let second = make_linear(vs.pp("second"), HIDDEN_DIM, LABELS)?;
+
+        Ok(Self { first, second })
+    }
+
+    fn forward(&self, image: &Tensor) -> Result<Tensor> {
+        let x = self.first.forward(image)?;
+        let x = x.relu()?;
+        self.second.forward(&x)
+    }
+}
+```
+
+Now, let's add the `candle-datasets` package to our project to access the MNIST dataset:
+
+```bash
+$ cargo add --git https://github.com/huggingface/candle.git candle-datasets
+```
+
+With the dataset available, we can implement our training loop:
+
+```rust
+use candle_core::{DType, Device, Result, Tensor, D};
+use candle_nn::{loss, ops, Linear, Module, Optimizer, VarBuilder, VarMap};
+
+fn training_loop(
+    m: candle_datasets::vision::Dataset,
+) -> anyhow::Result<()> {
+    let dev = Device::cuda_if_available(0)?;
+
+    let train_labels = m.train_labels;
+    let train_images = m.train_images.to_device(&dev)?;
+    let train_labels = train_labels.to_dtype(DType::U32)?.to_device(&dev)?;
+
+    // Initialize a VarMap to store trainable parameters
+    let varmap = VarMap::new();
+    let vs = VarBuilder::from_varmap(&varmap, DType::F32, &dev);
+    let model = Model::new(vs.clone())?;
+
+    let learning_rate = 0.05;
+    let epochs = 10;
+
+    // Initialize a stochastic gradient descent optimizer to update parameters
+    let mut sgd = candle_nn::SGD::new(varmap.all_vars(), learning_rate)?;
+    let test_images = m.test_images.to_device(&dev)?;
+    let test_labels = m.test_labels.to_dtype(DType::U32)?.to_device(&dev)?;
+    
+    for epoch in 1..epochs {
+        // Perform forward pass on MNIST data
+        let logits = model.forward(&train_images)?;
+        let log_sm = ops::log_softmax(&logits, D::Minus1)?;
+        
+        // Compute Negative Log Likelihood loss
+        let loss = loss::nll(&log_sm, &train_labels)?;
+
+        // Perform backward pass and update weights
+        sgd.backward_step(&loss)?;
+
+        // Evaluate model on test set
+        let test_logits = model.forward(&test_images)?;
+        let sum_ok = test_logits
+            .argmax(D::Minus1)?
+            .eq(&test_labels)?
+            .to_dtype(DType::F32)?
+            .sum_all()?
+            .to_scalar::<f32>()?;
+        let test_accuracy = sum_ok / test_labels.dims1()? as f32;
+        println!(
+            "{epoch:4} train loss: {:8.5} test acc: {:5.2}%",
+            loss.to_scalar::<f32>()?,
+            test_accuracy
+        );
+    }
+    Ok(())
+}
+```
+
+Finally, let's implement our main function:
+
+```rust
+pub fn main() -> anyhow::Result<()> {
+    let m = candle_datasets::vision::mnist::load()?;
+    return training_loop(m);
+}
+```
+
+Let's execute the training process:
+
+```bash
+$ cargo run --release
+
+> 1 train loss:  2.35449 test acc:  0.12%
+> 2 train loss:  2.30760 test acc:  0.15%
+> ...
+```