Remove some dead code after migration from python to C.

PiperOrigin-RevId: 213372027

1 files changed, 12 insertions, 0 deletions

diff --git a/tensorflow/c/eager/tape.h b/tensorflow/c/eager/tape.h
index ce038a4b57..49990b6249 100644
--- a/tensorflow/c/eager/tape.h
+++ b/tensorflow/c/eager/tape.h
@@ -440,6 +440,18 @@ Status InitialGradients(const VSpace<Gradient, BackwardFunction>& vspace,
   return Status::OK();
 }
 
+// TODO(agarwal): use an automatic mechanism for handling None arguments to
+// gradient functions.
+//
+// Some gradient functions can accept None arguments for gradients. The
+// following maps the operation name to the indices at which the corresponding
+// gradient function can accept None values. e.g. FusedBatchNorm outputs 5
+// values and hence receives 5 gradient values during backprop. However the
+// gradient function uses only the first of those values and ignores the rest.
+// The entry, "FusedBatchNorm": [1, 2, 3, 4], indicates that only the gradient
+// corresponding to index 0 is used, and the gradient values at indices 1-4 are
+// ignored (and hence can be None). The backprop algorithm can then leverage
+// this by not constructing zeros to pass for those indices.
 gtl::FlatMap<string, gtl::FlatSet<int>>* FunctionsAcceptingNoneForIndicesMap() {
   static auto* const m = new gtl::FlatMap<string, gtl::FlatSet<int>>({
       {"SoftmaxCrossEntropyWithLogits", {1}},