[device] find best logical mesh

[YuliangLiu0306]

What does this PR do

1. implement search_best_logical_mesh function, which could find best logical mesh for the given device list.

   The best logical mesh is searched in following steps:

   1. detect homogeneous device groups, we assume that the devices in the alpha_beta_dict are homogeneous if the beta value is close enough.
   2. Find the best homogeneous device group contains all the physical devices. The best homogeneous device group means the lowest beta value in the groups which contains all the physical devices. And the reason we require the group contains all the physical devices is that the devices not in the group will decrease the bandwidth of the group.
   3. If the best homogeneous device group is found, we will construct the largest ring for each device based on the best homogeneous device group, and the best logical mesh will be the union of all the rings. Otherwise, the best logical mesh will be the balanced logical mesh, such as shape (2, 2) for 4 devices.

   Usage:

       >>> physical_devices = [0, 1, 2, 3]
       >>> ab_profiler = AlphaBetaProfiler(physical_devices)
       >>> best_logical_mesh = profiler.search_best_logical_mesh()
       >>> print(best_logical_mesh)
       [[0, 1], [2, 3]]

2. implement extract_alpha_beta_for_device_mesh function which extract the mesh_alpha list and mesh_beta list based on the best logical mesh.

   Usage:

   	>>> physical_devices = [0, 1, 2, 3]
   	>>> ab_profiler = AlphaBetaProfiler(physical_devices)
   	>>> mesh_alpha, mesh_beta = profiler.extract_alpha_beta_for_device_mesh()
   	>>> print(mesh_alpha)
   	[2.5917552411556242e-05, 0.00010312341153621673]
   	>>> print(mesh_beta)
   	[5.875573704655635e-11, 4.7361584445959614e-12]

3. construct test cases to test above features.