Update docs (#2553)

* add module docs for candle-core

* doc each of the candle-nn modules and add the links to the doc page

candle-nn/src/var_map.rs

@@ -1,3 +1,5 @@
+//! A `VarMap` is a store that holds named variables.
+//!
 use candle::{DType, Device, Result, Shape, Tensor, Var};
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};