/*virtual*/ void ReduceElementsNode<ElemType>::BackpropTo(const size_t inputIndex, const FrameRange& fr) /*override*/
{
assert(inputIndex == 0), inputIndex;
// get the args
size_t rank = DetermineElementwiseTensorRank();
auto sliceOutputGrad = GradientTensorFor(rank, fr); // propagate from this one...
auto sliceInputGrad = InputRef(0).GradientTensorFor(rank, fr); // ...to this one
// gradients are not as simple as passing an op-code, unfortunately
switch (m_reductionOp)
{
case ElementWiseOperator::opSum:
// "Sum": broadcast the gradient
// "Mean": same as "Sum" with scaling by 1/#dims
sliceInputGrad.AddCopyOf(sliceOutputGrad, m_scale);
break;
case ElementWiseOperator::opLogSum:
{
auto input = InputRef(inputIndex).ValueTensorFor(rank, fr);
auto output = ValueTensorFor(rank, fr.AllowBroadcast());
// Let: f(x, y, z) = log(exp x + exp y + exp z)
// For the derivative we get:
// df / dx = exp(x)/exp(f)
// = exp(x – f)
sliceInputGrad.AddElementwiseProductWithExpOfDiffOf(sliceOutputGrad, input, output);
}
break;
case ElementWiseOperator::opMin:
case ElementWiseOperator::opMax:
auto input = InputRef(inputIndex).ValueTensorFor(rank, fr);
auto output = ValueTensorFor(rank, fr.AllowBroadcast());
// POTENTIAL PROBLEM:
// For ReduceMin/Max there are combinations of input values where the gradient is not defined because the function has an edge at these points.
// E.g. for ReduceMin this is the case when the minimum input value is attained by several inputs at the same time.
// In these cases there is no correct gradient.The question is if this could lead to any problems.
// Let's look at two scenarios where this might happen:
//
// * Scenario 1: The input comes from a layer of nodes like e.g. ReLU and some of them might operate in the regime where they clip to a constant value.
// In this case it's not a problem that the input gradient is kind of bad as the derivative of the concerning input nodes will be zero anyway.
//
// * Scenario 2: The input data is directly coming from training data. Here bad gradients don't matter as we wouldn't wan't to propagate gradients to the training data.
//
// So as we don't have a better solution yet and it probably doesn't have impact let's stay with the current solution.
// Also note that for Clip , Min, Max and ReLU we have the same kind of problem.
sliceInputGrad.AddCopyIfEqualOf(input, output, sliceOutputGrad);
break;
// more coming
}
}
/*virtual*/ void TraceNode<ElemType>::ForwardProp(const FrameRange& fr) /*override*/
{
size_t rank = DetermineElementwiseTensorRank();
auto result = ValueTensorFor(rank, fr);
auto input = InputRef(0).ValueTensorFor(rank, fr);
result.AssignCopyOf(input);
// do the tracing
Log(fr, false/*means log value*/);
}
/*virtual*/ void ReduceElementsNode<ElemType>::ForwardProp(const FrameRange& fr) /*override*/
{
// get the args
size_t rank = DetermineElementwiseTensorRank();
auto result = ValueTensorFor(rank, fr);
auto input = Input(0)->ValueTensorFor(rank, fr);
// the actual operation is a Copy with reduction, where the magic is in the reduction op
result.DoUnaryOpOf(0, input, 1, ElementWiseOperator::opCopy, m_reductionOp);
// note: we can implement "Mean" by passing 1/dim for alpha
}
/*virtual*/ void ReduceElementsNode<ElemType>::ForwardProp(const FrameRange& fr) /*override*/
{
// get the args
size_t rank = DetermineElementwiseTensorRank();
auto result = ValueTensorFor(rank, fr);
auto input = InputRef(0).ValueTensorFor(rank, fr);
// the actual operation is a Copy with reduction, where the magic is in the reduction op
// For "Mean", m_scale is 1/#elements, and 1 otherwise.
result.DoUnaryOpOf(0, input, m_scale, ElementWiseOperator::opCopy, m_reductionOp);
}
/*virtual*/ void ReduceElementsNode<ElemType>::BackpropTo(const size_t inputIndex, const FrameRange& fr) /*override*/
{
assert(inputIndex == 0), inputIndex;
// get the args
size_t rank = DetermineElementwiseTensorRank();
auto sliceOutputGrad = GradientTensorFor(rank, fr); // propagate from this one...
auto sliceInputGrad = Input(0)->GradientTensorFor(rank, fr); // ...to this one
// gradients are not as simple as passing an op-code, unfortunately
switch (m_reductionOp)
{
case ElementWiseOperator::opSum:
// "Sum": broadcast the gradient
sliceInputGrad.AddCopyOf(sliceOutputGrad);
break;
case ElementWiseOperator::opMax:
case ElementWiseOperator::opMin:
auto input = Input(inputIndex)->ValueTensorFor(rank, fr);
auto output = ValueTensorFor(rank, fr.AllowBroadcast());
// POTENTIAL PROBLEM:
// For ReduceMin/Max there are combinations of input values where the gradient is not defined because the function has an edge at these points.
// E.g. for ReduceMin this is the case when the minimum input value is attained by several inputs at the same time.
// In these cases there is no correct gradient.The question is if this could lead to any problems.
// Let's look at two scenarios where this might happen:
//
// * Scenario 1: The input comes from a layer of nodes like e.g. ReLU and some of them might operate in the regime where they clip to a constant value.
// In this case it's not a problem that the input gradient is kind of bad as the derivative of the concerning input nodes will be zero anyway.
//
// * Scenario 2: The input data is directly coming from training data. Here bad gradients don't matter as we wouldn't wan't to propagate gradients to the training data.
//
// So as we don't have a better solution yet and it probably doesn't have impact let's stay with the current solution.
// Also note that for Clip , Min, Max and ReLU we have the same kind of problem.
sliceInputGrad.AddCopyIfEqualOf(input, output, sliceOutputGrad);
break;
// more coming
// "LogPlus": softmax
// f(x) = log(sum_i exp x_i), hence gradient is:
// df / dx_i = 1 / (sum_j exp x_j) * exp x_i = (Softmax(x))_i = exp(x_i - ReduceLogPlus(x))
// targetGradient = gradientFromTop .* Exp (inputValue - outputValue) --TODO: verify
// i.e. compute dfference if input and output, then Exp in-place. No, would need temp memory. So needs its own opcode AddScaledExpOfDiff(). Ternary.
}
}
