Enhance in backward (#5262)

Set gradient's data type based on its forward variable

paddle/framework/backward.cc

@@ -18,6 +18,7 @@
 #include <deque>
 #include <list>
 #include <memory>
+#include <unordered_set>
 
 #include "paddle/framework/block_desc.h"
 #include "paddle/framework/op_registry.h"
@@ -285,6 +286,15 @@ static bool AllGradInSet(const std::vector<std::string>& names,
   return true;
 }
 
+static std::string FwdName(const std::string& grad_name) {
+  auto pos = grad_name.find("@GRAD");
+  if (pos == std::string::npos) {
+    return "";
+  } else {
+    return grad_name.substr(0, pos);
+  }
+}
+
 static void CreateGradVarInBlock(
     size_t grad_op_start_index,
     const std::unordered_map<std::string, std::string>& param_name_map,
@@ -294,6 +304,7 @@ static void CreateGradVarInBlock(
   for (size_t op_index = grad_op_start_index; op_index < ops.size();
        ++op_index) {
     bool need_infer_shape = false;
+    std::unordered_set<std::string> new_vars;
     ForEachVarName(ops[op_index]->Outputs(),
                    [&](const std::string& grad_var_name) {
                      if (block_desc->HasVar(grad_var_name)) {
@@ -301,8 +312,7 @@ static void CreateGradVarInBlock(
                      }
                      need_infer_shape = true;
                      auto var = block_desc->Var(grad_var_name);
-                     // FIXME(qiao) infer the datatype
-                     var->SetDataType(framework::DataType::FP32);
+                     new_vars.insert(var->Name());
                      auto it = param_name_map.find(grad_var_name);
                      if (it == param_name_map.end()) {
                        return false;
@@ -316,6 +326,21 @@ static void CreateGradVarInBlock(
                    });
     if (need_infer_shape) {
       ops[op_index]->InferVarType(block_desc);
+      for (auto& arg : ops[op_index]->OutputArgumentNames()) {
+        if (new_vars.find(arg) == new_vars.end()) {
+          continue;
+        }
+        auto pname = FwdName(arg);
+        auto* param = block_desc->FindVar(pname);
+        auto* grad = block_desc->FindVar(arg);
+        if (param == nullptr) {
+          LOG(WARNING) << "Cannot find forward variable of " << arg
+                       << ". Set its gradient to FP32";
+          grad->SetDataType(DataType::FP32);
+        } else {
+          grad->SetDataType(param->GetDataType());
+        }
+      }
       ops[op_index]->InferShape(*block_desc);
     }
   }