Pick an embedding size for `n` depending on `classes` if not given in `sz_dict`.

def def_emb_sz(classes, n, sz_dict=None):
    "Pick an embedding size for `n` depending on `classes` if not given in `sz_dict`."
    sz_dict = ifnone(sz_dict, {})
    n_cat = len(classes[n])
    sz = sz_dict.get(n, int(emb_sz_rule(n_cat)))  # rule of thumb
    return n_cat,sz