pybind/mgr/balancer/module.py: assign weight-sets to all buckets before balancing

pybind/mgr/balancer/module.py: assign weight-sets to all buckets before balancing

Add an additional check to make sure that the choose_args section has the same
number of buckets as the crushmap. If not, ensure that
get_compat_weight_set_weights assigns weight-sets to all buckets.

Without this change, if we end up with an orig_ws, which has fewer buckets
than the crushmap, the mgr will crash due a KeyError in do_crush_compat().

Fixes: https://tracker.ceph.com/issues/49576
Signed-off-by: Neha Ojha <nojha@redhat.com>