## Issue

In case there are multiple output values and one of them is loss, some
user reported the following error:
```
    output_grads[i] for i in outputs_with_grads_idxs
IndexError: tuple index out of range
...
RuntimeError: 
        Failed to run backward stage stage_backward for stage %submod_7 : [ = call_module[target=submod_7](args = (%submod_6, %_inputs), kwargs = {})
        Stage output: ('Tensor(torch.Size([100, 20, 4096]), grad=False)', 'Tensor(torch.Size([100, 4096]), grad=False)', 'Tensor(torch.Size([100, 4096]), grad=False)', 'Tensor(torch.Size([]), grad=True)', 'Tensor(torch.Size([100]), grad=False)', 'Tensor(torch.Size([100]), grad=False)')
        Output gradient: ('None',)
        Input: ['Tensor(torch.Size([100, 20, 4096]), grad=True)', 'Tensor(torch.Size([100, 20, 4096]), grad=False)', 'Tensor(torch.Size([100]), grad=False)', 'Tensor(torch.Size([100]), grad=False)']
```
Note this part: `Output gradient: ('None',)`

I can repro the issue in local_test_c10d_bwd.py, if I change the output
to:
```
-        return {"loss": loss}
+        return {"logits": x, "loss": loss}
```

## Cause
The above issue is caused by the fixed setting in the else case:
```
                # (None,) is for `stage_backward` signature
                bwd_kwargs["output_grads"] = (
                    grads if len(grads) > 0 else (None,)
                )
```
This tuple `(None,)` should have variable length depending on the
output.

## Fix
Only update `bwd_kwargs["output_grads"]` when we have actually received
gradients; otherwise, use the tuple prepared during IR phase, i.e.
`bwd_node.kwargs["output_grads"]`, which may look like `(None, None)` if
there are two outputs.

## Issue

In case there are multiple output values and one of them is loss, some
user reported the following error:
```
    output_grads[i] for i in outputs_with_grads_idxs
IndexError: tuple index out of range
...
RuntimeError: 
        Failed to run backward stage stage_backward for stage %submod_7 : [ = call_module[target=submod_7](args = (%submod_6, %_inputs), kwargs = {})
        Stage output: ('Tensor(torch.Size([100, 20, 4096]), grad=False)', 'Tensor(torch.Size([100, 4096]), grad=False)', 'Tensor(torch.Size([100, 4096]), grad=False)', 'Tensor(torch.Size([]), grad=True)', 'Tensor(torch.Size([100]), grad=False)', 'Tensor(torch.Size([100]), grad=False)')
        Output gradient: ('None',)
        Input: ['Tensor(torch.Size([100, 20, 4096]), grad=True)', 'Tensor(torch.Size([100, 20, 4096]), grad=False)', 'Tensor(torch.Size([100]), grad=False)', 'Tensor(torch.Size([100]), grad=False)']
```
Note this part: `Output gradient: ('None',)`

I can repro the issue in local_test_c10d_bwd.py, if I change the output
to:
```
-        return {"loss": loss}
+        return {"logits": x, "loss": loss}
```

## Cause
The above issue is caused by the fixed setting in the else case:
```
                # (None,) is for `stage_backward` signature
                bwd_kwargs["output_grads"] = (
                    grads if len(grads) > 0 else (None,)
                )
```
This tuple `(None,)` should have variable length depending on the
output.

## Fix
Only update `bwd_kwargs["output_grads"]` when we have actually received
gradients; otherwise, use the tuple prepared during IR phase, i.e.
`bwd_node.kwargs["output_grads"]`, which may look like `(None, None)` if
there are two outputs.