Implement Gather double backwards.

Author: gchanan

test/test_autograd.py

@@ -1891,7 +1891,6 @@ def unpack_variables(args):
 
 gradgradcheck_exclude_classes = set((
  'Cumprod',
- 'Gather',
  'Norm',
  'Prod',
 ))

torch/autograd/_functions/tensor.py

@@ -556,10 +556,9 @@ def forward(ctx, input, dim, index):
  return input.gather(dim, index)
 
  @staticmethod
- @once_differentiable
  def backward(ctx, grad_output):
- index, = ctx.saved_tensors
- grad_input = grad_output.new(ctx.input_size).zero_()
+ index, = ctx.saved_variables
+ grad_input = Variable(grad_output.data.new(ctx.input_size).zero_())
  return grad_input.scatter_add_(ctx.dim, index, grad_output), None, None