checkin basic var

python/tvm/var.py

+from __future__ import absolute_import as _abs
+from .expr import Expr
+
+class Var(Expr):
+    """Variables"""
+    def __init__(self, name, expr=None):
+        self.name = name
+        self.expr = expr
+
+    def children(self):
+        if self.expr is None:
+            return ()
+        return self.expr.children()