[IR][Pass] Refactor the fusion implementation

[yaoyaoding]

This PR refactor the implementation of post-scheduling fusion.

Previously, we use a TaskGraph to store the sub-graph inside each Task.

After this refactor, we remove the TaskGraph attribute of Task, but created a new kind of operator to represent the fused sub-graph. This allows us to support more kinds of fusion, and make the IR more clean.

Some other udpates:

1. Each lib.so will contains a single operator. We fix the name of packed function to 'launch' (previously, the name of packed function is the name of the task). This will avoid passing the function name and make the code more clean.
2. Now, the implement_cpu and implement_cuda methods of class Task can return a list of IRModule, indicating the tunable schedules.
3. Move the generation of 'launch' function to pass list. Now, we do not need to call the add_packed_func to manually add it.