Speed up NamedSharding construction.
* Compute the size of a mesh eagerly. We're almost always going to need this, because NamedSharding's constructor asks for it.
* Speed up mesh equality. It's likely we have only one mesh, and the identity equality test will hit. Do it first.
* don't call _prepare_axis_resources in ParsedPartitionSpec construction. This does a bunch of pointless tree flattening and list manipulation but we know we have exactly one PartitionSpec and can directly do the check we need, which is _check_unique_resources.
* only call _check_unique_resources on PartitionSpecs; it's easy to avoid doing it in other cases and then we don't need a bunch of isinstance checks.
* avoid use of collections.Counter when checking for unique resources. collections.Counter has a surprisingly slow isinstance test.
PiperOrigin-RevId: 724431847
