Add initial modules

.gitignore

@@ -1,2 +1,3 @@
-smolgrad/_build/*
-smolgrad/_opam/*
+_build/*
+_opam/*
+.vscode/*

dune-project

@@ -1,2 +1,2 @@
 (lang dune 2.0)
-(name smolgrad.ml)
+(name smolgrad-ml)

lib/dune

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+(library
+  (name nn)
+  (modules Neuron Network))

lib/neuron.ml

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+module Neuron = struct
+  type t = {
+    mutable data : float;
+    mutable grad : float;
+    mutable backward : unit -> unit;
+    op : string;
+    prev : t list;
+  }
+
+  let create data operator = { data; grad = 0.;
+    backward = (fun () -> ()); op = operator; prev = []
+  }
+
+  let add a b =
+    let out = create (a.data +. b.data) "+" in
+    out.backward <- (fun () ->
+      a.grad <- a.grad +. out.grad;
+      b.grad <- b.grad +. out.grad;
+    );
+    out
+
+  let mul a b =
+    let out = create (a.data *. b.data) "*" in
+    out.backward <- (fun () ->
+      a.grad <- a.grad +. b.data *. out.grad;
+      b.grad <- b.grad +. a.data *. out.grad;
+    );
+    out
+end

lib/neuron.mli

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+module Neuron : sig
+    type t
+
+    (* Constructor; constructs a unit neuron of a value and an operator. *)
+    val create : float -> string -> t
+
+    (* Adds two values, resulting in a new value. *)
+    val add : t -> t -> t
+
+    (* Multiplies two values, resulting in a new value. *)
+    val mul : t -> t -> t
+end