Another issue with _vjpConv2DBackpropInput

[sjaz24]

After fixing issue #329, I've encountered another issue during backpropagation that results in the following type of error:

Fatal error: Incompatible shapes: [1,32,32,64] vs. [4,4,32,64]: file /Users/stephenjohnson/Projects/Conv2dTransposeTest/.build/checkouts/swift-apis/Sources/TensorFlow/Bindings/EagerExecution.swift, line 299
Illegal instruction: 4

It took me a while to track down, but I believe it is because conv2DBackpropInput has differentiable order of wrt: (x, filter)

/// TensorFlow builtin conv2d gradient helper for the input.
@differentiable(wrt: (x, filter), vjp: _vjpConv2DBackpropInput)
@usableFromInline
func conv2DBackpropInput<Scalar: TensorFlowFloatingPoint>(

but _vjpConv2DBackpropInput is returning a tuple where conv2DBackpropFilter call is the first tuple item and the conv2D call is the second tuple item.

return (value, { v in
    (conv2DBackpropFilter(x, input: v, filterSizes: shape, strides: strides,
                          padding: padding, dilations: dilations),
     conv2D(v, filter: filter, strides: strides, padding: padding, dilations: dilations))

When I switch them around, everything appears to work fine. I'm going to submit a PR.

[rxwei]

Great catch!

[sjaz24]

Thank you!