Added the reverse, call from torch a chainer function

chainer_pytorch_migration/functions.py

@@ -66,3 +66,55 @@ def chainer_torch_function(torch_fn, inputs, *args, **kwargs):
     if len(y) == 1:
         return y[0]
     return y
+
+
+class TorchChainerFunction(torch.autograd.Function):
+    @staticmethod
+    def chainer_fn():
+        raise RuntimeError('chainer_fn function must be overriden')
+
+    @classmethod
+    def forward(cls, ctx, *inputs):
+        chainer_fn = cls.chainer_fn()
+        ctx.save_for_backward(*inputs)
+        c_inputs = tuple((chainer.Variable(cpm.asarray(x)) for x in inputs))
+        ctx.c_inputs = c_inputs
+        c_outputs = chainer_fn(*c_inputs)
+        if not type(c_outputs) is tuple:
+            c_outputs = (c_outputs,)
+        t_outputs = [cpm.astensor(y.array) for y in c_outputs]
+        for t_y in t_outputs:
+            t_y.requires_grad = True
+        ctx.c_outputs = c_outputs
+        return t_outputs[0]
+
+    @staticmethod
+    def backward(ctx, *grads):
+        grads = [ctx.c_outputs, ctx.c_inputs] + list(grads)
+        out_grads = _TorchChainerFunctionGrad.apply(*grads)
+        return out_grads
+
+
+class _TorchChainerFunctionGrad(torch.autograd.Function):
+
+    @staticmethod
+    def forward(ctx, *inputs):
+        c_outputs = inputs[0]
+        c_inputs = inputs[1]
+        inputs = inputs[2:]
+        ctx.save_for_backward(*inputs)
+        c_grads = tuple((chainer.Variable(cpm.asarray(g)) for g in inputs))
+        fwd_outputs = c_outputs
+        chainer.backward(fwd_outputs, c_grads, enable_double_backprop=True)
+        out_grads = tuple(
+            cpm.astensor(x.grad) for x in c_inputs
+        )
+        for t_y in out_grads:
+            t_y.requires_grad = True
+        ctx.c_outputs = [x.grad for x in c_inputs]
+        ctx.c_inputs = c_grads
+        return out_grads
+
+    def backward(ctx, *grads):
+        grads = [ctx.c_outputs, ctx.c_inputs] + list(grads)
+        return _TorchChainerFunctionGrad.apply(*grads)

tests/test_functions.py

@@ -47,3 +47,27 @@ def test_multiple_outputs():
     z.backward(torch.ones((3, 5)))
     t_grad = x.grad
     assert torch.allclose(t_grad, cpm.astensor(c_grad))
+
+
+def test_torch_chainer_function():
+    class TorchChainerSigmoid(cpm.functions.TorchChainerFunction):
+        @staticmethod
+        def chainer_fn():
+            return chainer.functions.sigmoid
+    # Combined torch
+    x = torch.ones(10)
+    x.requires_grad = True
+    y = torch.sin(x)
+    y = TorchChainerSigmoid.apply(y)
+    y = torch.sum(y)
+    y.backward()
+    ct_grad = x.grad
+
+    # All in torch
+    x = torch.ones(10)
+    x.requires_grad = True
+    y = torch.sin(x)
+    y = torch.sigmoid(y)
+    y = torch.sum(y)
+    y.backward()
+    assert torch.allclose(ct_grad, x.grad)