chore(mge): minor improvements related to grad

GitOrigin-RevId: 102467d79d148b52f4dfefadeb3e6a7d7a0d2ad6

imperative/python/src/grad.cpp

@@ -155,17 +155,7 @@ struct BackwardGraphWithClosure {
         }
         if (null_grad) return;
 
-        ApplyContext ctx;
-        ctx.op = backward_graph->backward;
-        ctx.flags = is_tracing ? Flags::TRACE : 0;
-        ctx.nargs = nargs;
-        ctx.args = args;
-        for (size_t i = 0; i < nargs; ++i) {
-            ctx.flags |= args[i]->m_flags;
-            mgb_assert(args[i]);
-        }
-
-        auto igrads = apply(ctx);
+        auto igrads = apply(backward_graph->backward, args, nargs);
         auto&& it = igrads.begin();
         for (auto [i, p] : views::enumerate(backward_graph->input_has_grad)) {
             if (p) {

imperative/python/src/tensor.h

@@ -252,6 +252,18 @@ auto apply(std::shared_ptr<OpDef> op, T&& tensors)
     return apply(ctx);
 }
 
+inline auto apply(std::shared_ptr<OpDef> op, Tensor*const* args, size_t nargs) {
+    ApplyContext ctx;
+    ctx.op = std::move(op);
+    ctx.flags = is_tracing ? Tensor::Flags::TRACE : 0;
+    ctx.nargs = nargs;
+    ctx.args = args;
+    for (size_t i = 0; i < nargs; ++i) {
+        ctx.flags |= args[i]->m_flags;
+    }
+    return apply(ctx);
+}
+
 void init_tensor(pybind11::module);
 
 extern PyObject *cpp_apply_with_tracing, *cpp_apply_compiled_mode;

imperative/src/impl/backward_graph_opt.cpp

@@ -111,4 +111,8 @@ OptimizedBackwardGraphResult::OptimizedBackwardGraphResult(const BackwardGraphRe
             }
         }
     }
+
+    if (!fgraph.outputs.size()) {
+        precomp.reset();
+    }
 }

imperative/src/impl/ops/backward_graph.cpp

@@ -9,7 +9,11 @@
  * "AS IS" BASIS, WITHOUT ARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  */
 
+#include <sstream>
+#include <range/v3/all.hpp>
+
 #include "megbrain/imperative/ops/backward_graph.h"
+#include "megbrain/imperative/ops/opr_attr.h"
 #include "../op_trait.h"
 
 namespace mgb {
@@ -66,6 +70,67 @@ std::tuple<SmallVector<LogicalTensorDesc>, bool> BackwardGraph::InternalGraph::i
     return {ret, validated};
 }
 
+std::string BackwardGraph::InternalGraph::repr() {
+    std::ostringstream buf;
+    buf << "(";
+    for (size_t i = 0; i < inputs.size(); ++i) {
+        if (i > 0) buf << ", ";
+        buf << "%" << inputs[i];
+    }
+    buf << ") => {\n";
+    auto fmt_const = [](size_t i, TensorPtr& t) {
+        if (t->shape().ndim == 1 && t->shape()[0] == 1) {
+            auto&& v = t->get_value();
+            if (v.dtype() == dtype::Float32{}) {
+                return std::to_string(*v.ptr<dt_float32>());
+            } else if (v.dtype() == dtype::Int32{}) {
+                return std::to_string(*v.ptr<int32_t>());
+            }
+        }
+        return std::string("%c") + std::to_string(i);
+    };
+    std::unordered_map<size_t, std::string> const_reps;
+    for (auto&& [i, t] : constants) {
+        const_reps.emplace(i, fmt_const(i, t));
+    }
+    for (auto& [op, ins, outs] : exprs) {
+        buf << "  ";
+        if (outs.size()) {
+            for (size_t i = 0; i < outs.size(); ++i) {
+                if (i > 0) buf << ", ";
+                buf << "%" << outs[i];
+            }
+            buf << " = ";
+        }
+        if (auto* p = op->try_cast_final<OprAttr>()) {
+            buf << p->type;
+        } else {
+            buf << op->dyn_typeinfo()->name;
+        }
+        for (size_t i : ins) {
+            buf << " ";
+            auto&& it = const_reps.find(i);
+            if (it != const_reps.end()) {
+                buf << it->second;
+            } else {
+                buf << "%" << i;
+            }
+        }
+        buf << "\n";
+    }
+    buf << "  ";
+    if (outputs.size()) {
+        for (size_t i = 0; i < outputs.size(); ++i) {
+            if (i > 0) buf << ", ";
+            buf << "%" << outputs[i];
+        }
+    } else {
+        buf << "()";
+    }
+    buf << "\n}\n";
+    return buf.str();
+}
+
 MGB_DYN_TYPE_OBJ_FINAL_IMPL(BackwardGraph);
 
 namespace {

imperative/src/include/megbrain/imperative/ops/backward_graph.h

@@ -71,6 +71,8 @@ public:
             }
             return ret;
         }
+
+        std::string repr();
     };
 
     const InternalGraph& graph() const {
@@ -93,6 +95,8 @@ public:
         return false;
     }
 
+    std::string repr() {return m_graph.repr();}
+
 private:
     InternalGraph m_graph;
 };