Add masked_grad kw argument to to_dense (#96095)
As in the title.
The `masked_grad` kw argument is required for `to_dense` backward to distinguish the expected semantics of sparse tensors. `masked_grad=True` means that the `to_dense` backward will apply a mask to the returned gradient where the mask is defined by the input indices. The default semantics implies `masked_grad==True` for BC but see the [comment](https://github.com/pytorch/pytorch/pull/96095/files#diff-d4df180433a09071e891d552426911c227b30ae9b8a8e56da31046e7ecb1afbeR501-R513) in `to_dense_backward`.
As a consequence, existing code that is run through autograd engine must replace `.to_dense()` calls with `.to_dense(masked_grad=False)`. For example,
```python
torch.autograd.gradcheck(lambda x: torch.sum(x, [0]).to_dense())
torch.autograd.gradcheck(lambda x: torch.sparse.sum(x, [0]).to_dense())
```
(recall, gradcheck has `masked=False` as default) must be updated to
```python
torch.autograd.gradcheck(lambda x: torch.sum(x, [0]).to_dense(masked_grad=False))
torch.autograd.gradcheck(lambda x: torch.sparse.sum(x, [0]).to_dense(masked_grad=True), masked=True)
```
Fixes https://github.com/pytorch/pytorch/issues/95550
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96095
Approved by: https://github.com/cpuhrsch
