Add the candle-datasets crate (#322)

* Move the vision datasets to a separate crate.

* Move the batcher bits.

* Update the readme.

* Move the tiny-stories bits.

---------

Co-authored-by: Jane Doe <jane.doe@example.org>

candle-datasets/src/nlp/tinystories.rs

@@ -0,0 +1,122 @@
+//! Helper functions for the tinystories dataset. This uses the pre-tokenized version as generated
+//! by the tools from https://github.com/karpathy/llama2.c
+use candle::{Device, Result, Tensor};
+
+pub struct Dataset {
+    valid_tokens: Vec<memmap2::Mmap>,
+    train_tokens: Vec<memmap2::Mmap>,
+}
+
+fn mmap_file(p: &std::path::PathBuf) -> Result<memmap2::Mmap> {
+    let file = std::fs::File::open(p)?;
+    let mmap = unsafe { memmap2::MmapOptions::new().map(&file)? };
+    Ok(mmap)
+}
+
+impl Dataset {
+    pub fn new<P: AsRef<std::path::Path>>(dir: P) -> Result<Self> {
+        let dir = dir.as_ref();
+        let mut bin_files = vec![];
+        for file in std::fs::read_dir(dir)?.flatten() {
+            let file = file.path();
+            if let Some(extension) = file.extension() {
+                if extension == "bin" {
+                    bin_files.push(file)
+                }
+            }
+        }
+        if bin_files.len() < 2 {
+            candle::bail!("found less than two bin files in {:?}", dir)
+        }
+        bin_files.sort();
+        let valid_tokens = mmap_file(&bin_files[0])?;
+        let train_tokens = bin_files[1..]
+            .iter()
+            .map(mmap_file)
+            .collect::<Result<Vec<_>>>()?;
+        Ok(Self {
+            valid_tokens: vec![valid_tokens],
+            train_tokens,
+        })
+    }
+
+    pub fn train_tokens(&self) -> usize {
+        self.train_tokens.len()
+    }
+
+    pub fn valid_tokens(&self) -> usize {
+        self.valid_tokens.len()
+    }
+}
+
+pub struct DatasetRandomIter<'a> {
+    all_tokens: &'a [memmap2::Mmap],
+    tokens: Vec<&'a memmap2::Mmap>,
+    current_tokens: &'a memmap2::Mmap,
+    indexes_in_bytes: Vec<usize>,
+    seq_len: usize,
+    device: Device,
+}
+
+impl<'a> DatasetRandomIter<'a> {
+    pub fn new(ds: &'a Dataset, valid: bool, seq_len: usize, device: Device) -> Self {
+        use rand::seq::SliceRandom;
+        use rand::thread_rng;
+
+        let all_tokens = if valid {
+            &ds.valid_tokens
+        } else {
+            &ds.train_tokens
+        };
+        let mut tokens = all_tokens.iter().collect::<Vec<_>>();
+        tokens.shuffle(&mut thread_rng());
+        let current_tokens = tokens.pop().unwrap();
+        let seq_len_in_bytes = seq_len * 2;
+        let mut indexes_in_bytes = (0..current_tokens.len() - seq_len_in_bytes)
+            .step_by(seq_len_in_bytes)
+            .collect::<Vec<_>>();
+        indexes_in_bytes.shuffle(&mut thread_rng());
+        Self {
+            all_tokens,
+            tokens,
+            current_tokens,
+            indexes_in_bytes,
+            seq_len,
+            device,
+        }
+    }
+}
+
+impl<'a> Iterator for DatasetRandomIter<'a> {
+    type Item = Result<(Tensor, Tensor)>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        use byteorder::{LittleEndian, ReadBytesExt};
+        use rand::seq::SliceRandom;
+        use rand::thread_rng;
+
+        let seq_len = self.seq_len;
+        if self.indexes_in_bytes.is_empty() {
+            if self.tokens.is_empty() {
+                self.tokens = self.all_tokens.iter().collect();
+                self.tokens.shuffle(&mut thread_rng());
+            }
+            self.current_tokens = self.tokens.pop().unwrap();
+            let seq_len_in_bytes = self.seq_len * 2;
+            self.indexes_in_bytes = (0..self.current_tokens.len() - seq_len_in_bytes)
+                .step_by(seq_len_in_bytes)
+                .collect::<Vec<_>>();
+            self.indexes_in_bytes.shuffle(&mut thread_rng());
+        }
+        let start_idx = self.indexes_in_bytes.pop().unwrap();
+        let bytes = &self.current_tokens[start_idx..start_idx + 2 * (seq_len + 1)];
+        let mut tokens = vec![0u16; bytes.len() / 2];
+        if let Err(err) = std::io::Cursor::new(bytes).read_u16_into::<LittleEndian>(&mut tokens) {
+            return Some(Err(err.into()));
+        }
+        let tokens = tokens.into_iter().map(|v| v as u32).collect::<Vec<_>>();
+        let inputs = Tensor::new(&tokens[..seq_len], &self.device);
+        let targets = Tensor::new(&tokens[1..], &self.device);
+        Some(candle::error::zip(inputs, targets))
+    }
+}