change file structure to use LowererImplImperative and move ir_generators.h to correct include/ dir

Author: weiya711

src/ir/ir_generators.h renamed to include/taco/ir/ir_generators.h

@@ -1,4 +1,4 @@
-#include "ir_generators.h"
+#include "taco/ir/ir_generators.h"
 
 #include "taco/ir/ir.h"
 #include "taco/error.h"

include/taco/lower/lowerer_impl.h

@@ -11,10 +11,11 @@
 
 #include "taco/ir/ir.h"
 #include "taco/ir/simplify.h"
-#include "ir/ir_generators.h"
+#include "taco/ir/ir_generators.h"
 #include "taco/ir/ir_printer.h"
 
 #include "taco/lower/lowerer_impl.h"
+#include "taco/lower/lowerer_impl_imperative.h"
 #include "taco/lower/iterator.h"
 #include "mode_access.h"
 
@@ -33,7 +34,7 @@ namespace taco {
 
 
 // class Lowerer
-Lowerer::Lowerer() : impl(new LowererImpl()) {
+Lowerer::Lowerer() : impl(new LowererImplImperative()) {
 }
 
 Lowerer::Lowerer(LowererImpl* impl) : impl(impl) {