Fix ZeroPad2d backwards with negative pads.

Author: gchanan

test/test_nn.py

@@ -3562,6 +3562,12 @@ def add_test(test):
         constructor_args=((1, 2, 3, 4),),
         input_size=(2, 3, 4, 4)
     ),
+    dict(
+        module_name='ZeroPad2d',
+        constructor_args=((-1, -1, -1, -2),),
+        input_size=(2, 3, 4, 4),
+        desc='negative_dims'
+    ),
     dict(
         module_name='ConstantPad2d',
         constructor_args=((1, 2, 3, 4), 2),

torch/nn/_functions/padding.py

@@ -45,21 +45,24 @@ def backward(ctx, grad_output):
         pad_l, pad_r, pad_t, pad_b = ctx.pad
 
         grad_input = Variable(grad_output.data.new(ctx.input_size).zero_())
-        grad_input_slices = [slice(0, x + 1,) for x in ctx.input_size]
+        grad_input_slices = [slice(0, x,) for x in ctx.input_size]
 
         def narrow_slice(dim, start, length):
             grad_input_slices[dim] = (slice(grad_input_slices[dim].start + start,
-                                            grad_input_slices[dim].start + start + length + 1))
+                                            grad_input_slices[dim].start + start + length))
+
+        def slice_length(dim):
+            return grad_input_slices[dim].stop - grad_input_slices[dim].start
 
         #  crop grad_input if necessary
         if pad_t < 0:
-            narrow_slice(2, -pad_t, grad_input.size(2) + pad_t)
+            narrow_slice(2, -pad_t, slice_length(2) + pad_t)
         if pad_b < 0:
-            narrow_slice(2, 0, grad_input.size(2) + pad_b)
+            narrow_slice(2, 0, slice_length(2) + pad_b)
         if pad_l < 0:
-            narrow_slice(3, -pad_l, grad_input.size(3) + pad_l)
+            narrow_slice(3, -pad_l, slice_length(3) + pad_l)
         if pad_r < 0:
-            narrow_slice(3, 0, grad_input.size(3) + pad_r)
+            narrow_slice(3, 0, slice_length(3) + pad_r)
 
         # crop grad_output if necessary
         cg_output = grad_output