Expose functools.reduce initializer argument to tree_util.tree_reduce (#2935)

* Expose `functools.reduce` initializer argument to `tree_util.tree_reduce`.

`functools.reduce` takes an optional `initializer` argument (default=None) which is currently not exposed by `tree_reduce'. This can be useful e.g. for computing an L2 penalty, where you would initialize with 0., and then sum the L2 for each parameter.

Example:
```
def l2_sum(total, param):
  return total + jnp.sum(param**2)

tree_reduce(l2_sum, params, 0.)
```

* Only call functools.reduce with initializer when it is not None.

* Change logic to check for number of args to allow None value as initializer

* Rename seq to tree, and add tree_leaves

* Change reduce to functools.reduce.

* Make tree_reduce self-documenting

* Replace jax.tree_leaves with tree_leaves

* Update to use custom sentinel instead of optional position argument

* jax.tree_leaves -> tree_leaves

Author: bastings

jax/tree_util.py

@@ -231,8 +231,12 @@ def _replace_nones(sentinel, tree):
     else:
       return tree
 
-def tree_reduce(f, tree):
-  return functools.reduce(f, tree_leaves(tree))
+no_initializer = object()
+def tree_reduce(function, tree, initializer=no_initializer):
+  if initializer is no_initializer:
+    return functools.reduce(function, tree_leaves(tree))
+  else:
+    return functools.reduce(function, tree_leaves(tree), initializer)
 
 def tree_all(tree):
   return all(tree_leaves(tree))