Add missing initializer argument of 0 to tree_reduce in tree_vdot and tree_sum.

[carlosgmartin]

Currently, the following commands

$ python3 -c "import optax; optax.tree_utils.tree_sum({})"
$ python3 -c "import optax; optax.tree_utils.tree_vdot({}, {})"

yield the following error:

TypeError: reduce() of empty iterable with no initial value

This PR fixes this bug by setting initializer=0 in the call to tree_reduce inside each of these functions.

[vroulet]

Thanks! Can you add a quick test too? (just to bookkeep such edge cases in our tests in case we make a new similar function)

[carlosgmartin]

@vroulet Done.

[vroulet]

Thanks!

[fabianp]

thanks for the contribution @carlosgmartin !

do you mind to explicitly use the keyword argument ? that is, use

 jtu.tree_reduce(operator.add, vdots, initializer=0)

instead of the current jtu.tree_reduce(operator.add, vdots, 0). They're semantically equivalent but the first one is easier to read and less error-prone

[carlosgmartin]

@fabianp Done.