Upgrade NAS-API to v2.0:

we use an abstract class NASBenchMetaAPI to define the spec of an API; it can be inherited to support different kinds of NAS API, while keep the query interface the same.

configs/nas-benchmark/hyper-opts/200E.config

@@ -0,0 +1,13 @@
+{
+  "scheduler": ["str",   "cos"],
+  "eta_min"  : ["float", "0.0"],
+  "epochs"   : ["int",   "200"],
+  "warmup"   : ["int",   "0"],
+  "optim"    : ["str",   "SGD"],
+  "LR"       : ["float", "0.1"],
+  "decay"    : ["float", "0.0005"],
+  "momentum" : ["float", "0.9"],
+  "nesterov" : ["bool",  "1"],
+  "criterion": ["str",   "Softmax"],
+  "batch_size": ["int", "256"]
+}