[Target] Add "features" property to Target (#12121)

This adds a generated property "features" to the `Target` which can
contain a read-only list of available features in line with
https://github.com/apache/tvm-rfcs/pull/78.

Features are re-generated upon parsing into a `Target` object rather than being
attached as `attrs`. The `Target` JSON is therefore stored without the
inferred `features` attached.

python/tvm/_ffi/runtime_ctypes.py

@@ -216,6 +216,7 @@ class Device(ctypes.Structure):
         "stackvm": 1,
         "cpu": 1,
         "c": 1,
+        "test": 1,
         "hybrid": 1,
         "composite": 1,
         "cuda": 2,

python/tvm/target/target.py

@@ -43,6 +43,14 @@ class TargetKind(Object):
         return dict(_ffi_api.ListTargetKindOptionsFromName(kind_name))
 
 
+class TargetFeatures:
+    def __init__(self, target):
+        self.target = target
+
+    def __getattr__(self, name: str):
+        return _ffi_api.TargetGetFeature(self.target, name)
+
+
 @tvm._ffi.register_object
 class Target(Object):
     """Target device information, use through TVM API.
@@ -207,6 +215,10 @@ class Target(Object):
     def libs(self):
         return list(self.attrs.get("libs", []))
 
+    @property
+    def features(self):
+        return TargetFeatures(self)
+
     def get_kind_attr(self, attr_name):
         """Get additional attribute about the target kind.