[Relay] Add compiler pass tutorial docs (#2746)

* Add Relay compiler pass tutorial docs

* Add Python API hook wrapping step

* Incorporate feedback

* More doc iteration

* Mooooore iteration

* Rewrite `runtime.md` in rst

docs/dev/index.rst

@@ -31,4 +31,5 @@ In this part of documentation, we share the rationale for the specific choices m
    hybrid_script
    relay_intro
    relay_add_op
+   relay_add_pass
    codebase_walkthrough

src/relay/pass/fold_constant.cc

@@ -35,8 +35,11 @@ using FInterpreter = runtime::TypedPackedFunc<Value(Expr)>;
 
 class ConstantChecker : private ExprVisitor {
  public:
-  // Check whether an expression is constant. The results are memorized.
+  // Check whether an expression is constant. The results are memoized.
   bool Check(const Expr& expr) {
+    // The `ConstantNode` case is common enough that we check directly for the
+    // case here, to avoid the time overhead of dispatching through the vtable
+    // and the space overhead of memoizing always-true results.
     if (expr.as<ConstantNode>()) {
       return true;
     }
@@ -44,7 +47,7 @@ class ConstantChecker : private ExprVisitor {
     if (it != memo_.end())
       return it->second;
     VisitExpr(expr);
-    return memo_[expr];  // return memorized result or the default value false
+    return memo_[expr];  // return memoized result or the default value false
   }
 
  private: