add usage

Author: YuliangLiu0306

colossalai/device/alpha_beta_profiler.py

@@ -15,6 +15,18 @@
 class AlphaBetaProfiler:
     '''
     Profile alpha and beta value for a given device list.
+
+    Usage:
+        # Note: the environment of execution is supposed to be
+        # multi-process with multi-gpu in mpi style.
+        >>> physical_devices = [0, 1, 4, 5]
+        >>> ab_profiler = AlphaBetaProfiler(physical_devices)
+        >>> ab_dict = profiler.profile_ab()
+        >>> print(ab_dict)
+        {(0, 1): (1.9641406834125518e-05, 4.74049549614719e-12), (0, 4): (1.9506998360157013e-05, 6.97421973297474e-11), (0, 5): (2.293858677148819e-05, 7.129930361393644e-11),
+         (1, 4): (1.9010603427886962e-05, 7.077968863788975e-11), (1, 5): (1.9807778298854827e-05, 6.928845708992215e-11), (4, 5): (1.8681809306144713e-05, 4.7522367291330524e-12),
+         (1, 0): (1.9641406834125518e-05, 4.74049549614719e-12), (4, 0): (1.9506998360157013e-05, 6.97421973297474e-11), (5, 0): (2.293858677148819e-05, 7.129930361393644e-11),
+         (4, 1): (1.9010603427886962e-05, 7.077968863788975e-11), (5, 1): (1.9807778298854827e-05, 6.928845708992215e-11), (5, 4): (1.8681809306144713e-05, 4.7522367291330524e-12)}
     '''
 
     def __init__(self,