Simplify torch.nn.grad by calling into aten::convolution_backward (#81839)
`torch.nn.grad` has its own implementations of gradients for conv1d, conv2d, and conv3d. This PR simplifies them by calling into the unified `aten::convolution_backward` backend instead.
The existing implementation of conv2d_weight is incorrect for some inputs (see issue #51430). This PR fixes the issue.
This PR expands coverage in test_nn to include conv1d_weight, conv2d_weight, and conv3d_weight, which were previously untested. It also expands the cases for conv2d to cover issue #51430.
Fixes #51430
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81839
Approved by: https://github.com/albanD
