[Relay][Frontend] Keras Support (apache#2336)

python/tvm/relay/frontend/__init__.py

@@ -8,3 +8,4 @@
 from __future__ import absolute_import
 
 from .mxnet import from_mxnet
+from .keras import from_keras

python/tvm/relay/frontend/common.py

@@ -1,5 +1,6 @@
 """Common utilities"""
 from __future__ import absolute_import as _abs
+from .. import expr as _expr
 
 
 class RequiredAttr(object):
@@ -181,8 +182,6 @@ def get_int_list(self, key, default=RequiredAttr()):
             raise AttributeError("Required attribute {} not found.".format(key))
         return default
 
-
-
     def get_bool(self, key, default=RequiredAttr()):
         """Get bool tuple attribute
 
@@ -204,3 +203,27 @@ def get_bool(self, key, default=RequiredAttr()):
         if isinstance(default, RequiredAttr):
             raise AttributeError("Required attribute {} not found.".format(key))
         return default
+
+
+class ExprTable(object):
+    """Table storing Relay expressions by names."""
+    def __init__(self):
+        self.exprs = {}
+        self.params = {}
+        self.const_ctr = 1
+
+    def new_const(self, value, shape=None, dtype="float32"):
+        name = "_param_%d" % (self.const_ctr)
+        if hasattr(value, "shape"):
+            shape = value.shape
+        self.const_ctr += 1
+        self.params[name] = value
+        self.exprs[name] = _expr.var(name_hint=name, shape=shape, dtype=dtype)
+        return self.exprs[name]
+
+    def get_expr(self, name):
+        return self.exprs[name]
+
+    def set_expr(self, name, expr):
+        assert isinstance(expr, _expr.Expr)
+        self.exprs[name] = expr