[autoparallel] refactored shape consistency to remove redundancy

[FrankLeeeee]

This PR refactored the usage of shape consistency manager. The details are described below.

Problem

Previously, each handler and strategy constructor requires a shape consistency manager object as input for initialization. However, this manager is only used in calculating the resharding cost and nowhere else. Meanwhile, calculating resharding cost is implemented twice in the op handler and strategy constructor. After moving this to utils, the handler and constructor do not need to keep the manager as a member any more.

Fix

The shape consistency manager is now a singleton class where there will only be one instance in the global namespace. The usage of resharding is now purely via function call (i.e. generate_resharding_cost and set_shape_consistency_manager_options, even though the latter is not implemented yet). The handler and constructor only need to call these functions now.

Side note

1. Besides, I found the previous PR has an inconsistent way of refactoring compared to this one, thus I also updated the usage of generate_sharding_spec.
2. Each op handler now has a new argument handle_backward. This is to configure whether backward stats should be computed, can be useful for inference as backward is not needed.