[Relay][Hashing] Structural hash - incorporate the var type into its hash (#3267)

Currently, the BindVar function does not take Var type into account. This causes
two same graph structures with different var shapes to have same hash.
Structural hash is used for keeping track of which operators we have
already compiled. Because of this, two operators with different shapes end up
pointing to same compiled code. The failure is encountered at runtime, where the
expected input shape asserts are not met.

src/relay/ir/hash.cc

@@ -219,6 +219,9 @@ class RelayHashHandler:
   size_t BindVar(const NodeRef& var) {
     size_t hash = std::hash<int>()(var_counter++);
     CHECK_EQ(hash_map_.count(var), 0);
+    if (auto var_node = var.as<VarNode>()) {
+      hash = Combine(hash, TypeHash(var_node->type_annotation));
+    }
     hash_map_[var] = hash;
 
     const auto* ty_param = var.as<TypeVarNode>();

tests/python/relay/test_pass_alpha_equal.py

@@ -594,7 +594,24 @@ def test_graph_equal():
     # Check the difference in the text format.
     assert not alpha_equal(z0, z3)
 
+def test_hash_unequal():
+    x1 = relay.var("x1", shape=(10, 10), dtype="float32")
+    y1 = relay.var("y1", shape=(10, 10), dtype="float32")
+    func1 = relay.Function([x1, y1], relay.add(x1, y1))
 
+    # func2 is exactly same structure with same variables shapes and dtypes
+    x2 = relay.var("x2", shape=(10, 10), dtype="float32")
+    y2 = relay.var("y2", shape=(10, 10), dtype="float32")
+    func2 = relay.Function([x2, y2], relay.add(x2, y2))
+
+    assert ir_pass.structural_hash(func1) == ir_pass.structural_hash(func2)
+
+    # func3 is same as func1 but with different var shapes
+    x3 = relay.var("x3", shape=(20, 10), dtype="float32")
+    y3 = relay.var("y3", shape=(20, 10), dtype="float32")
+    func3 = relay.Function([x3, y3], relay.add(x3, y3))
+
+    assert not ir_pass.structural_hash(func1) == ir_pass.structural_hash(func3)
 
 if __name__ == "__main__":
     test_tensor_type_alpha_equal()
@@ -617,3 +634,4 @@ if __name__ == "__main__":
     test_op_alpha_equal()
     test_var_alpha_equal()
     test_graph_equal()
+    test_hash_unequal()