Fix composable checkpoint(use_reentrant=True) with multi args (#103590)

The `_ModuleHookCheckpointFunction.backward()` should take in `*output_grads` instead of `output_grads`. Otherwise, we may see an error like:
```
TypeError: backward() takes 2 positional arguments but 5 were given
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103590
Approved by: https://github.com/rohan-varma, https://github.com/fduwjj, https://github.com/fegin

test/distributed/_composable/test_checkpoint.py

@@ -4,6 +4,7 @@ import unittest
 from collections import deque
 from contextlib import ContextDecorator
 from copy import deepcopy
+from typing import Tuple
 
 import torch
 import torch.nn as nn
@@ -66,6 +67,32 @@ class RandomModel(nn.Module):
         return torch.matmul(x, y)
 
 
+class MultiOutputModel(nn.Module):
+    def __init__(self, device: torch.device):
+        super().__init__()
+        self.w1 = nn.Parameter(torch.randn((100, 100), device=device))
+        self.w2 = nn.Parameter(torch.randn((100, 100), device=device))
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        z = x @ self.w1
+        z = nn.functional.relu(z)
+        z = z @ self.w2
+        return z.sin(), z.cos()
+
+
+class MultiInputModel(nn.Module):
+    def __init__(self, device: torch.device):
+        super().__init__()
+        self.w = nn.Parameter(torch.randn((100, 100), device=device))
+
+    def forward(self, xs: Tuple[torch.Tensor, torch.Tensor]) -> torch.Tensor:
+        assert len(xs) == 2, f"Expects 2 args but got {len(xs)}"
+        x, y = xs
+        z = x + y
+        z = z @ self.w
+        return nn.functional.relu(z)
+
+
 class TestCheckpoint(TestCase):
     def _get_graph_size(self, out: torch.Tensor) -> int:
         q = deque([out.grad_fn])
@@ -143,6 +170,29 @@ class TestCheckpoint(TestCase):
         for p1, p2 in zip(net1.parameters(), net2.parameters()):
             self.assertEqual(p1.grad, p2.grad)
 
+    @parametrize("use_reentrant", [True, False])
+    def test_multi_args(self, use_reentrant: bool):
+        """
+        Tests checkpoint for modules with multiple output args and hence
+        multiple backward function input args.
+        """
+        device = torch.device("cpu")
+        net1 = nn.Sequential(
+            MultiOutputModel(device),
+            MultiInputModel(device),
+            MultiOutputModel(device),
+            MultiInputModel(device),
+        )
+        net2 = deepcopy(net1)
+        checkpoint(net2[0], use_reentrant=use_reentrant)
+        checkpoint(net2[2], use_reentrant=use_reentrant)
+        x1 = torch.randn(20, 100, requires_grad=True)
+        x2 = x1.clone()
+        net1(x1).sum().backward()
+        net2(x2).sum().backward()
+        for p1, p2 in zip(net1.parameters(), net2.parameters()):
+            self.assertEqual(p1.grad, p2.grad)
+
 
 instantiate_parametrized_tests(TestCheckpoint)
 

torch/distributed/_composable/checkpoint_activation.py

@@ -47,7 +47,7 @@ class _ModuleHookCheckpointFunction(torch.autograd.Function):
         return output
 
     @staticmethod
-    def backward(ctx, output_grads: Tuple[Optional[torch.Tensor]]) -> Any:  # type: ignore[override]
+    def backward(ctx, *output_grads: Tuple[Optional[torch.Tensor]]) -> Any:  # type: ignore[override]
         if not torch.autograd._is_checkpoint_valid():
             raise RuntimeError(
                 "Checkpointing is not compatible with .grad() or when an "